The present invention relates to the treatment of tumours, especially to the treatment of melanoma and cancer of the colon, and to the circumvention of ultidrug resistance in cancer treatments.

For certain types of cancer, chemotherapy has been capable of rendering patients with responsive tumours free of disease. However, this responsive category does not include the most frequently encountered forms of malignant tumours.

The most common types of cancer in western popula¬ tions are colon, lung and breast cancer. Each of these can be treated to some extent with existing chemotherapy, with different drugs being used preferentially for each type of malignancy (for instance, doxorubicin, cyclophos- phamide and methotrexate for breast cancer, 5-fluoro- uracil for colon cancer) , but response rates are not good. In addition, melanoma is a disease which is increasing in incidence at an alarming rate among fair- skinned populations. In melanoma, only 25-30% of patients with disseminated disease respond to treatment, and only 5-10% sustain durable remission (Evans B.D., et al., Proc. Am. Soc. Clin. Oncol. 1990, 9, 276).

There is therefore a great need for new types of cancer therapy, and a desperate need for such treatments for the above cancers in particular.

The basis for the development of the majority of anticancer drugs used today has been a panel of mouse tumours including transplantable leukaemias, the Lewis lung carcinoma and the colon 38 adenocarcinoma. A number of human tumour xenografts in mice have also been used. In general, the leukaemias are the most sensitive to experimental agents, the xenografts are the most resis¬ tant and the Lewis lung and colon 38 are of intermediate sensitivity (Goldin A., et al., Eur. J. Cancer 1981, 17, 129-142).

The murine Lewis lung adenocarcinoma is a tumour which initially arose spontaneously in C ₅₇ B1 mice and which has a number of features which make it a good model for clinical carcinomas. It grows easily both in vitro and in vivo, and is aneuploid, heterogeneous, metastatic and resistant to many but not all clinical antitumour agents. Zacharski (Zacharεki L.R., Haemostasis 1986, 16, 300-320) has concluded that although Lewis lung has the cytological appearance of a large-cell cancer, its rapid rate of growth, propensity to cause lethal meta- stases, as well as its susceptibility to combination chemotherapy, radiation and anticoagulant treatment, make it a good model for human small-cell lung cancer (SCLC) . The colon 38 tumour arose in carcinogen-treated mice, and because it is sensitive to 5-fluorouracil it can be considered as a useful model for human colon cancer (Corbett T.H., et al., Cancer Chemother. Rep. 1975, 5,

169-186, and Cancer 1977, 40, 2660-2680). Human melanoma xenografts have been considered for some time as an appropriate model for the development of new anticancer drugs for melanoma (Taetle R. , et al., Cancer 1987, 60, 1836-1841) .

The essence of treating cancer with cytotoxic anticancer drugs is to combine a mechanism of cyto- toxicity with a mechanism of selectivity for tumour cells over host cells. The selectivity of a drug for a particular cancer will depend on the expression by that cancer of properties which promote drug action, and which differ from tumour to tumour.

Most of the currently available cytotoxic drugs can be broadly divided into four groups: those which react chemically with DNA (such as the alkylating agents and cisplatin and bleomycin) , those which disrupt DNA synthesis (such as the antimetabolites) , those which disrupt the mitotic apparatus (such as the Vinca alka¬ loids and taxol) and those which are directed against the cellular enzyme topoisomerase II ("topo II") in order to effect changes in the topological form of the DNA.

DNA topoisomerases were named after the first method used to detect their activity. When incubated with closed circles of double-stranded DNA prepared from viruses or bacteria, topoisomerases enzymatically change the number of coils contained in each circle (circular forms of DNA with different degrees of coiling are called

topo-isomers) . The topoisomerases are perhaps better understood as enzymes which temporarily break one strand of the DNA double helix (topoisomerase I or "topo I") or which simultaneously break two strands of the DNA double helix ("topo II") in order to effect changes in the topological form of the DNA.

Topoisomerases have two main functions in the cell. The first is to act as swivel points on the DNA in association with DNA and RNA polymerases during the biosynthesis of nucleic acids required for cell replica¬ tion and gene expression. The second is to untangle the DNA strands of the daughter chromosomes following DNA replication prior to cell division. The DNA of chromo¬ somes is organised as a series of loops on a protein- aceous "scaffold". After duplication of chromosomes and of the "scaffolds", the DNA loops must be separated. Since there are hundreds of thousands of DNA loops attached to each chromosome scaffold it is not hard to imagine the necessity for an enzyme which effectively removes tangles by passing one double DNA strand through another. This process absolutely requires topo II.

Topo I acts by transiently breaking a DNA strand and attaching itself to one of the free ends of the broken DNA via the amino acid tyrosine. Topo II contains two identical protein subunits, each of which is capable of breaking a DNA strand and attaching itself to one of the free ends. With both DNA strands broken, a second DNA

double helix can be allowed to pass between the two enzyme protein subunits, thus allowing not only swivel¬ ling but also untangling of DNA. The process is normally spontaneously reversible by cleavage of the enzyme-DNA links and re-sealing of DNA breaks to restore the DNA to its original form.

Topo II-directed agents include a number of impor¬ tant clinical anti-cancer drugs such as anthracycline antibiotics (e.g. doxorubicin) , epipodophyllotoxin derivatives (e.g. etoposide) and synthetic DNA inter¬ calating drugs (e.g. amsacrine) . These act by jamming the enzyme in its DNA-associated form (Liu L.F., Annu. Rev. Biochem. 1989, 58, 351-375). Such molecular lesions might be expected to be innocuous, since the drug eventually dissociates itself from the complex and the DNA strand breaks are then repaired perfectly. However, in a small proportion of cases, the presence of drug causes the complex to be dissociated abnormally, generat¬ ing some kind of DNA lesion which eventually leads to cell death.

Although the antitu our activity of many of these agents has been known for many years, it is only since 1984 that the molecular target of action has been identified (Nelson E.M. , Tewey K.M. , Liu L.F., Proc. Natl. Acad. Sci. USA 1984, 81, 1361-1364 and Tewey K.M. , et al., Science 1984, 226, 466-468).

A number of mechanisms of resistance to topo II

poisons have now been identified, and in many cases the development of resistance to one drug is accompanied by the simultaneous acquisition of resistance to a variety of other drugs. Since the mechanism of resistance determines the pattern of cross-resistance to other drugs, an understanding of these processes is of great importance to the strategy for the use of these agents. Several resistance mechanisms important to the use of these agents have now been characterised in experimental systems, including those involved in drug transport (Endicott J.A. , Ling V., Annu. Rev. Biochem 1989, 58, 351-375) , drug-target interaction (Beck W.T. , Biochem, Pharmacol. 1987, 36, 2879-2888) and drug detoxification (Deffie A.M., et al., Cancer Res. 1988, 48, 3595-3602). Attempts to overcome multidrug resistance (mdr) clinically have been concerned mainly with the first of these mechanisms, a drug transport mechanism that pumps drug out of cells. Various inhibitors of this process, such as verapamil, are known and some have been used in combination with drugs such as doxorubicin and etoposide to treat cancer (Stewart D.J. , Evans W.K., Cancer Treat. Rev. 1989, 16, 1-10, Judson I.R., Eur. J. Cancer 1992, 28, 285-289).

Another approach is to design drugs which can overcome mdr. We have now discovered that the investiga- tional drug acridine carboxamide ("DACA") appears to be one such drug.

The compound tested was the dihydrochloride of N-[2- (dimethylamino)ethyl]acridine-4-carboxamide of formula

and is described and claimed in EP 98098 (USP 4590277) . That patent also describes and claims other acridine car- boxamide compounds and their use for the treatment of tumours; more particularly, the treatment of Lewis lung tumours and leukaemia is described.

Various other derivatives of DACA have been tested for their antitu our activity and the results are reported in the literature. Active compounds are carboxamides having an unsubstituted or substituted aromatic ring system comprising two or more fused rings and having an oxygen or an aromatic nitrogen peri to the carboxamide side chain. As well as the other acridine carboxamides (EP 98098/USP 4590277 and Denny W.A. , et al., J. Med. Chem. 1987; 30: 658-663), examples are phenyl quinoline and pyrido quinoline carboxamides (EP 206802 A/USP 4904659, Atwell G.J., et al., J. Med. Chem. 1988; 31: 1048-1052 and Atwell G.J. , et al., J. Med. Chem. 1989; 32: 396-401) , phenazine carboxamides (EP 172744 A/US Application 765993 filed 1985 and Rewcastle G.W., et al., J. Med. Chem. 1987; 30: 843-851),

carboxamides having angular tricyclic chromophores: phenanthridine carboxamides (NZ Patent 215286, 1986 and Atwell G.J., et al., J. Med. Chem. 1988; 31: 774-779) and carboxamides having various linear tricyclic chro o- phores, including dibenzo[l,4]dioxin carboxamides (Palmer B.D., et al., J. Med. Chem. 1988; 31: 707-712, Lee et al., J. Med. Chem. 1992; 35, 258-266, Palmer et al, J. Org. Chem. 1990; 55: 438-441, Lee et al., J. Chem. Soc, Perkin Trans, l, 1990, 1071-1074, and Ca bie et al. , J. Organo et. Chem., 1991: 420, 387 ff.). These other compounds are structurally very similar to DACA and are able to act in the same way.

DACA is a DNA-binding drug which acts at the same target, topoisomerase II, as do drugs such as amsacrine and etoposide. We have now found, however, that it has a different in vitro cytotoxicity profile to these com¬ pounds and a number of advantages over existing clinical drugs in the class of topoisomerase-directed agents.

Firstly, it is active against cell lines displaying both P-glycoprotein-mediated or "transport" resistance and "atypical" or "altered" multidrug resistance; in this respect it is unique among topo II inhibitors.

DACA and related compounds may therefore be used to circumvent mdr. For this it may be used in combination with other cytotoxic drugs, more especially those that kill cells by a different cytotoxic mechanism, and/or as a second-line treatment if first- line treatment fails

because of the development of multidrug resistance.

The present invention further provides a use of DACA or other fused-ring aromatic carboxamide having an aromatic nitrogen atom or an oxygen atom peri to the carboxamide side chain or a physiologically tolerable acid addition salt thereof, for the manufacture of a medicament for overcoming multidrug resistance in the treatment of tumours.

The present invention further provides a pharma- ceutical preparation comprising

(i) DACA or other fused-ring aromatic carboxamide having an aromatic nitrogen atom or an oxygen atom peri to the carboxamide side chain or a physiologically tolerable acid addition salt thereof, and

(ii) a non-topo II-inhibiting cytotoxic drug, in admixture or conjunction with a pharmaceutically suitable carrier.

The present invention also provides a combined preparation for use in the treatment of cancer, comprising separate components (i) and (ii) above for simultaneous or sequential administration.

Secondly, we have found that, unexpectedly, DACA is effective against advanced colon 38 tumours and an advanced melanoma xenograft in mice colon when adminis¬ tered in a divided dose schedule over a period of two hours. (In this context "advanced tumour" means that the

tumour was more than 5 mm in diameter at the time of measurement.) In contrast, a single administration of DACA at the maximum tolerated dose (150 mg/kg) , which is curative against Lewis lung tumours growing as lung nodules in mice, was only marginally effective.

Thirdly, DACA has the ability to cross the blood brain barrier, suggesting that rapidly growing brain tumours may also be treatable, more particularly when administered in a divided dose schedule. Accordingly, the present invention further provides the use of DACA or other fused-ring aromatic carboxamide having an aromatic nitrogen atom or an oxygen atom peri to the carboxamide side chain or a physiologically toler¬ able acid addition salt thereof, for the manufacture of a medicament for the treatment of detectable colon cancer or of melanoma, more especially by administration of a divided dose, the constituent doses being administered at frequent intervals.

There may, for example, be at least 2 administra- tions in total in the divided dose, administrations being preferably at least every 2 hours, for example every hour or every hour, for up to 4 hours or longer.

Thus, the present invention also provides the use of DACA or other tumour-inhibiting fused-ring aromatic carboxamide having an aromatic nitrogen atom or an oxygen atom peri to the carboxamide side chain or a physiologi¬ cally tolerable acid addition salt thereof, for the

manufacture of a divided-dose medicament for the treat¬ ment of tumours, including melanoma, brain tumours and colon tumours, by a divided-dose treatment regime comprising two or more administrations at frequent intervals, preferably by 2 to 4 constituent doses being administered over a period of up to 4 hours, for example 2 to 4 hours.

We believe also that suitable DNA-binding compounds will reduce the toxicity of DACA when administered in conjunction with a divided high-dose schedule of DACA. DNA-binding compounds include, for example 9-amino- acridine; such compounds have the ability to inhibit the antitumour activity of DACA.

Thus, preferably, a DNA-binding agent is used in combination with at least one of the constituent doses of DACA or other fused-ring aromatic carboxamide to reduce the host toxicity of DACA or specified other compound, the combination of drugs being adminstered together or sequentially. Doses of DACA in mice of 100 to 300 mg/kg, especially 150 to 250 mg/kg, more especially substan¬ tially 200 mg/kg, administered as a divided dose over a period of 2 to 4 hours, have proved suitable. Adminis¬ tration to humans of, for example, substantially 800 mg/m ² of DACA or equivalent amount of other carboxamide, should be mentioned, but lower or higher amounts may also be possible. As explained above, advantageous results

are obtained when the dose is administered as a divided dose, producing a high plasma level followed by a drop in level and then a high level again. Thus, the use of substantially 800 mg/m ² for the total of the constituent doses of a divided dose should be mentioned.

In one embodiment the divided-dose regime comprises a second administration after an interval from the first administration so as to maintain a content of active ingredient in the blood but after the level of active ingredient has dropped, for example to a value in the range of from 5 to 30% of the maximum level reached after the first administration, and optionally one or more further administrations at the corresponding interval from the earlier administration; such sub- sequent administrations when the plasma level has reached a value in the range of from 10 to 50% of the immediately preceding maximum should be mentioned.

Compounds suitable for use according to the present invention are those of the general formula

ArCONH(CH ₂ ) _n Y (I)

in which

Ar represents an unsubstituted or substituted aromatic ring which is part of a fused ring system which includes one or more further aromatic rings, and wherein the carboxamide side chain is peri to a

nitrogen atom that is part of an aromatic ring in the fused ring system or to an oxygen atom in a non- aromatic or aromatic ring in the fused ring system, Y represents C(NH)NH ₂ , NHC(NH)NH ₂ or NR4R5, where each of R ₄ and R ₅ separately is H or lower alkyl optionally substituted by one or more of the same or different substituents selected from hydroxy, lower alkoxy and amino functions, or R ₄ and R ₅ together with the nitrogen atom to which they are attached form a 5- or 6-membered heterocyclic ring optionally containing a further hetero atom; and n represents an integer from 2 to 6, and their physiologically tolerable acid addition salts and N-oxides thereof. Ar may represent, for example, an unsubstituted or substituted fused ring system selected from a linear fused tricyclic ring system in which at least 2 outer rings are aromatic, a phenanthridine ring system, and a fused bicyclic ring system in which both rings are aromatic, and wherein the carboxamide side chain is peri to a nitrogen atom that is part of an aromatic ring in the fused ring system or to an oxygen atom in a non- aromatic or aromatic ring in the fused ring system.

The ring system may comprise, for example, three fused aromatic rings, preferably linear, or two fused aromatic rings with a carbocyclic or heterocyclic aromatic ring as substituent. Of the fused aromatic

rings, one or more may be heterocyclic.

Thus, for example, Ar may be an acridine, phenazine, or phenanthridine ring system or a quinoline ring system substituted in the 2-position by a phenyl or pyridyl ring, or dibenzo[l,4]dioxin ring system.

Thus, the compound may have the general formula

CONH(CH ₂ ) _n Y

in which X is peri to the carboxamide side chain and in which ring A is unsubstituted or substituted and one of the methine groups (-CH=) may optionally be replaced by an aza (-N=) group, and

(i) X represents -N= and B completes a 6-membered aromatic ring fused to ring A and which is optionally fused to a further aromatic ring, or (ii) X represents O and B completes a 5- or 6-membered ring fused to ring A and which is fused to a further aromatic ring to form a linear fused ring system, and in either case (i) or (ii) the resulting fused ring or ring system B may be unsubstituted or substituted.

In a preferred embodiment of the present invention there is used a compound of the general formula

in which

R-^ represents H, CH ₃ or NHR ₀ , where R ₀ is H, COCH ₃ , S0 ₂ CH ₃ , COPh, S0 ₂ Ph or lower alkyl optionally substituted with hydroxy, lower alkoxy and/or amino functions;

R ₂ represents H or lower alkyl, halogen, CF ₃ , CN,

S0 ₂ CH ₃ , N0 ₂ , OH, NH ₂ , NHS0 ₂ R ₃ , NHCOR ₃ , NHCOOR ₃ , 0R ₃ , SR ₃ , NHR ₃ or NR ₃ R ₃ (where R ₃ is lower alkyl optionally substituted with hydroxy, lower alkoxy and/or amino functions) , and/or may represent the substitution of an aza (-N=) group for one of the methine (-CH=) groups in the carbocyclic ring, Y represents C(NH)NH ₂ , NHC(NH)NH ₂ or NR ₄ R ₅ , where each of R ₄ and R ₅ separately is H or lower alkyl optionally substituted with hydroxy, lower alkoxy and/or amino functions, or R ₄ and R ₅ together with the nitrogen atom *"o which they are attached form a 5- or 6-membered heterocyclic ring optionally containing a further hetero atom; n represents an integer from 2 to 6; X± represents H, and

X ₂ represents a phenyl or pyridyl ring unsubstituted or substituted by a substituent Rg, or

X _l and X ₂ , together with the carbon atoms to which they are attached, form a fused benzene ring unsub- stituted or substituted by a substituent Rg, and

Rg represents lower alkyl, halogen, CF ₃ , CN, S0 ₂ CH ₃ , N0 ₂ , OH, NH ₂ , NHS0 ₂ R ₃ , NHCOR ₃ , NHC00R ₃ , OR ₃ , SR ₃ , NHR ₃ or NR ₃ R (where R ₃ is lower alkyl optionally substituted with hydroxy, lower alkoxy and/or amino functions) ; or a phenyl ring optionally further substituted by lower alkyl, halogen, CF ₃ , CN, S0 ₂ CH ₃ , N0 ₂ , OH, NH ₂ , NHCOR ₃ , NHCOOR ₃ , OR ₃ , SR ₃ , NHR ₃ or NR R ₃ (where R is lower alkyl optionally substituted with hydroxy, lower alkoxy and/or amino functions) ; and/or may represent the substitution of an aza (-N=) group for one of the methine (-CH=) groups in the ring; or a physiologically tolerable acid addition salt, or, especially when X- _j _ = H and X ₂ is unsubstituted or substituted phenyl or pyridyl, a 1-N-oxide thereof. Alternatively,

R _j -L and X^ complete a fused carbocyclic aromatic ring, forming a phenanthridine ring system, and

X ₂ represents H or Rg is as specified above and wherein when Rg represents a phenyl ring or a phenyl ring in which an aza group replaces a methine group (i.e. R ₆ represents a pyridyl group) , there may be up to two of

the specified other substituents which may be the same or different, and

R represents H or up to two of the R ₂ substituents (which may be the same or different) specified above and/or may represent the substitution of an aza group for one of the methine groups in the carbocyclic ring.

A compound of the general formula Ia in which X± and X ₂ complete a fused ring and R-^ represents an unsub¬ stituted or substituted phenyl group should also be mentioned.

Another class of compounds is, for example, rep¬ resented by the general formula lb

in which

R ₇ represents H or up to three substituents, at positions selected from 2 to 4 and 6 to 9, wherein any two or all of the substituents may be the same or different and the substituents are selected from lower alkyl radicals; lower alkyl radicals substituted by one or more of the same or different substituents selected from hydroxy, lower alkoxy and/or amino functions; OH; SH; 0CH ₂ Ph; OPh; N0 ₂ ; halogen; CF ₃ ; amino; NHS0 ₂ R ₃ , NHCOR ₃ , NHCOOR ₃ , OR ₃ and SR ₃ (where R ₃ has the meaning

given above; and CONH(CH ₂ ) _n ιY' (where n' and Y' are as defined below) , there being a maximum of one CONH(CH ₂ ) _n ιY' group; or any two of R ₇ at adjacent positions represent -CH=CH-CH=CH- as part of an extra benzene ring or -0-CH ₂ -0- (methylenedioxy) and the third of R has any one of the meanings given above with the exception of an OH at position 2;

Y and Y ¹ , which may be the same or different, each has the meaning given above for Y; and n and n', which may be the same or different, each has the meaning given above for n; and physiologically tolerable acid addition salts, 5- and 10- mono-N-oxides and 5,10-di-N-oxides thereof.

A further class of compounds is R ₇ -substituted dibenzo[l,4]dioxin-l-carboxamides, where R7 has the meanings given above, especially 9-substituted compounds which optionally may also be further substituted as specified.

These compounds may be prepared by methods known per se, for example by methods described in the relevant literature references mentioned above and in EP 98098 A (US 4590277), in EP 206802 A (US 4904659) and in EP 172744 A (US Application 765993 filed 1985) or by analogous methods. When used herein, the term "lower alkyl" denotes an alkyl group having from 1 to 5, preferably 1 to 4, carbon atoms.

An amino function as substituent of a lower alkyl radical represented by any of R ₃ , R ₄ , R5, R ₀ and R ₇ may be unsubstituted or, for example, substituted by one or two lower alkyl groups (where lower alkyl has the meaning given above) , especially by one or two methyl groups. Thus, for example, an amino substituent of a lower alkyl radical represented by R ₃ , R , R ₅ , R ₀ and/or R ₇ may be NH ₂ , NHCH ₃ or N(CH ₃ ) ₂ .

A lower alkoxy group as substituent of a lower alkyl radical represented by R ₃ , R ₄ , R ₅ , R ₀ and/or R ₇ is especially a methoxy group.

A heterocyclic radical represented by R ₄ and R ₅ and the nitrogen atoms to which they are attached may, if desired, contain an additional hetero atom, and is 5- or 6-membered. An example is a morpholino group.

Examples of optionally substituted lower alkyl groups include those substituted by hydroxy, lower alkoxy or an amino function, for example lower alkyl optionally substituted with hydroxy, amino, methylamino, dimethyl- amino or methoxy. when X-^ + X complete a fused benzene ring such lower alkyl groups are preferably unsubstituted or substituted with hydroxy and/or amino groups.

In a NR ₃ R ₃ group the two R ₃ substituents may be the same or different, but are preferably the same. A preferred class of compound of the above formula I where X represents H and X ₂ represents a phenyl or pyridyl ring is where R-^ represents H, and, more

especially,

R ₂ represents H,

Y represents N(CH ₃ ) ₂ , n represents 2 and if X ₂ represents a pyridyl ring, that ring is unsubsti¬ tuted, and if X ₂ represents a phenyl ring that ring is unsubstituted or substituted by halogen, N0 or OCH ₃ .

A pyridyl ring represented by X ₂ is preferably a 4- pyridyl ring.

The use of an acridine carboxamide of the general formula

where R^- and n have the meanings given above, R ₈ represents H or up to two of the groups CH ₃ , OCH ₃ , halogen, CF ₃ , N0 ₂ , NH ₂ , NHC0CH ₃ , and NHC00CH ₃ placed at positions 1-3 and 5-8, and/or may represent the substitution of an aza (-N=) group for one of the methine (-CH=) groups in the carbocyclic ring; and

Y represents C(NH)NH ₂ , NHC(NH)NH ₂ , or NR ₄ R ₅ , where each of R ₄ and R ₅ is H or lower alkyl optionally

substituted with hydroxyl and/or amino groups; and where any lower alkyl radical has up to 4 carbon atoms, and the physiologically tolerable acid addition salts thereof, should especially be mentioned.

A preferred subclass of these compounds of formula I* are those where R _! represents H or NH ₂ , Rg represents up to two of 1-, 5-, 6-, 7- and 8-N0 ₂ , 5- and 6-CH ₃ , and 5-Cl,

Y represents NHC(NH)NH ₂ , N(CH ₃ ) ₂ , or NHCH ₂ CH ₂ 0H, and n represents 2.

Compounds specifically identified in EP 98098 A, EP 206802 A and EP 172744 A and in the literature references given above should also be mentioned, includ¬ ing not only DACA, but also N-[2-(dimethylamino)ethyl]-9- methoxyphenazine-1-carboxamide, N-[2-(dimethylamino)- ethyl]-2-(4-pyridyl)quinoline-8-carboxamide, N-[2- (dimethylamino)ethyl]-2-phenylquinoline-8-carboxamide and N-[2-(dimethylamino)ethyl]-9-bromo-dibenzo[1,4]dioxin-1- carboxamide. The latter compound and other substituted dibenzo[l,4]dioxin-l-carboxamides are disclosed in J. Med. Chem. 1992, 35, 258-266.

When any of R ₂ , R ₆ and R ₈ represents the substitu- tion in the ring of an aza group for one of the methine groups, that ring may be unsubstituted or substituted as specified above.

Compounds of the general formula Ia or I' in which Rg or R ₈ represents the substitution of an aza group for one of the methine groups and which optionally contains a further Rg or R ₈ substituent(s) should also be mentioned.

The compounds used according to the invention, including compounds of formulae Ia and lb, form pharma¬ ceutically acceptable addition salts with both organic and inorganic acids. Examples of suitable acids for salt formation are hydrochloric, sulphuric, phosphoric, acetic, citric, oxalic, malonic, salicylic, malic, fumaric, succinic, ascorbic, maleic and methanesulphonic acids.

When used as a means of combating mdr, in combina- tion with another cytotoxic drug, for example a DNA- reactive agent, a DNA-synthesis inhibitor or an agent which disrupts the mitotic apparatus, or any other category of agent that disrupts cellular metabolism, the compound of the general formula I may be administered together with, or sequentially with (before or after) the other cytotoxic drug. Thus, DACA could be given, for example, up to ± 2 days of a second drug, or alterna¬ tively could be given as a separate or alternating course with another cytotoxic drug and separated by a period of bone marrow or other host tissue recovery, generally 3 to 4 weeks. DACA may, for example, be administered by intravenous infusion using the divided dose regime

mentioned above, for example as a series of 2 to 4 administrations over a period of 2 to 4 hours.

Suitable DNA-reactive agents are, for example, cisplatin, cyclophosphamide, bleomycin, carboplatin, decarbazine, mitomycin C and melphalan. Suitable DNA synthesis inhibitors are, for example, 5-fluorouracil, 5- fluorodeoxyuridine and methotrexate; and suitable agents that disrupt the mitotic apparatus are, for example, taxol and suitable Vinca alkaloids, for example, vincris- tine, vinblastine and vindesine. Other non-topo II inhibitors that may be used include, for example, tamoxifin (which acts by preventing the action/synthesis of hormones) , tissue necrosis factor, Interleukin II and Interferon. These agents should be used in a treatment schedule which has been found optimal for antitumour effect; for example, cyclophosphamide may be used at monthly intervals, and vincristine at monthly or weekly intervals.

There are four main types of multidrug resistance related to topo II inhibitor:

(a) No change in the topoisomerase II enzyme but increased transport of the drug out of the cell.

DACA is not susceptible to this, while etoposide and doxorubicin are. (b) No change in the topoisomerase II enzyme, but an increase in drug detoxifying enzymes. This applies to doxorubicin but not to etoposide or DACA.

(c) A quantitative change (decrease) in the amount of topoisomerase II enzyme. DACA, etoposide and doxorubicin are equally susceptible, since DNA damage depends on the amount of enzyme present. Since the amount of topo II is regulated during the cell division cycle, cytokinetic resistance, whereby non-cycling cells resist the effects of topo II agents, may involve this type of resistance.

(d) A qualitative change in the topoisomerase II enzyme, a result either of a switch in gene expression

(there are two genes for topoisomerase II and one is normally dominant) or of a mutation in the gene, or of a change in modification of the enzyme after it has been synthesised. This results in a change in drug-target interaction. The qualitative change is accompanied by a differential change in sensitivity: in general, cells become highly resistant to amsacrine, moderately resistant to etoposide and doxorubicin, and, we have ascertained, minimally resistant to DACA.

Investigation of the cytotoxicity of DACA under conditions of continuous drug exposure in a variety of human and mouse cell lines and in a panel of 60 human cell lines revealed IC ₅₀ values (defined as drug con- centration required over approx. 5 cell doubling times for the reduction of the final cell culture density by 50%) ranging from 0.09 μM to 3.4μM, and a mean IC50 value

for human cell lines of 1.3μM. The latter value compared with 2 μM for the 4-pyridyl-quinoline analogue, 0.76 μM for amsacrine, 0.1 μM for the amsacrine analogue 4 '-(9- [4-[N-methylcarboxamido]-5-methyl]-acridinylamino)- methanesulphon-m-anisidide ("CI-921") and 81 μM for etoposide. Whereas the patterns of cytotoxicity of amsacrine, CI-921 and etoposide in the human cell line panel were very similar, those of DACA and its pyrido- quinoline analogue were quite different, suggesting differences in mode of action.

A multidrug resistant subline of P388 murine leukaemia (P/ACTD) was tested for sensitivity to DACA. This line was cross-resistant to actinomycin D, doxo¬ rubicin, mitoxantrone, etoposide and vincristine. Its resistance to vincristine was overcome by the presence of verapamil (10 μM) . It stained for the presence of P- glycoprotein, consistent with the presence of transport- mediated multidrug resistance. This line was sensitive to DACA in vitro and in vivo, suggesting that DACA may be useful in at least some types of multidrug resistance. DACA was also able to overcome, to a large extent, other mechanisms of multidrug resistance, as demonstrated in a series of sublines of Jurkat leukaemia cells which were highly resistant to amsacrine, etoposide and doxorubicin. Two of these lines had been selected for resistance to amsacrine, and were more than 100-fold cross-resistant to amsacrine but only 2- to 4-fold

cross-resistant to DACA. These lines exhibited resis¬ tance mechanisms which were distinguishable from trans¬ port-mediated multidrug resistance. We believe that this ability to overcome resistance mechanisms accounts for the different IC ₅₀ patterns observed with the human cell line panels.

It is apparent that in many tumours, regrowth during therapy is associated with resistance. The type of resistance is not yet properly characterised, but if it involves the mechanisms discussed above, DACA may be useful for second time treatment, especially in the divided dose regime mentioned above.

In connection with the circumventing of multidrug resistance especially, there should especially be mentioned carboxamides in which the fused rings of the ring system are all aromatic.

The use of all the above compounds and combinations in the treatment of sarcoma and of lung, breast, ovarian and testicular cancer and of melanoma and advanced colon cancer and brain tumours should especially be mentioned. The use of compounds of the general formula I to treat colon tumours has been suggested previously, but there has been no evidence of their suitability for this treatment and there has been no indication that they are effective in test systems even with delay of initiation of treatment beyond day 2 or 3 after tumour implantation, as is usual in tests. There has also been

no disclosure of high activity against such tumours. Such high activity would not have been expected since the most closely structurally related topo II inhibitor, amsacrine, is inactive. An initial experiment against advanced colon 38 tumour (on day 11 after implantation) , using the same schedule of administration as used for Lewis lung (3 injections at 4-day intervals) gave only a modest growth delay (4 days) . However, by adjustment of the administration schedule of DACA, growth delay of the advanced colon 38 tumour (5-8 mm in diameter) was increased to more than 21 days. Thus, while intermittent schedules (270 mg/kg q ⁴ days x 3; 400 mg/kg q ⁷ days x 3) provided only modest growth delays (3 days and 7 days, respectively) , repeated injection schedules (4 injections at 30 minute intervals; 180 + 120 + 120 + 120 μmol/kg, q ⁷ days x 3) provided a 21 day growth delay. Such results were completely unexpected. We have found that a low drug concentration for a long time (for example 6 hours) is much more toxic than a high concentration for a correspondingly shorter time. We believe these unusual "self-inhibitory" properties of DACA may be of help in the new divided dose administra- tion strategy. Because DACA diffuses more slowly in solid tumours such as colon tumours, than in normal tissues, peak drug concentrations in tumours are lower

than in normal tissues: an obvious disadvantage. However, because, as we have found, higher concentrations of DACA are less inhibitory than lower ones, the adjust¬ ment of the dosing strategy may provide partial protec- tion of normal tissues.

The second and optional further administrations of the constituent doses of the divided-dose schedule of the present invention are at substantially more frequent intervals than are those of individual doses in conven- tional therapy - monthly, weekly or daily, for example - differing, for example, by an order of magnitude.

The second administration may be commenced, for example, at least 30 minutes, e.g. at least 45 min or at least 1 hr, but up to 8 hr, after commencement of the first administration; intervals of 30 min and of up to 1 hr, up to 90 min, up to 2 hr, up to 3 hr, up to 4 hr, up to 5 hr or up to 6hr should be mentioned. Any third or further administration may also, for example, be com¬ menced at least 30 min, e.g. at least 45 min or at least 1 hr, and up to 8 hr after the beginning of the previous administration. As with the first interval, intervals of 30 min, and of up to l hr, up to 90 min, up to 2 hr, up to 3 hr, up to 4 hr, up to 5 hr or up to 6 hr should be mentioned. In the case of three or more administrations the intervals between two successive administrations may be the same or different.

Each constituent dose of the divided dose of the

present invention may be administered by whatever method of administration is appropriate, for example by intra¬ venous infusion over a period or by administration all at once; the amount of active ingredient in the different constituent doses may be the same or different. The or each interval of non-administration may be the same or different, for example in the range of from 15 ins to 8 hr, usually at least 30 min and often at least 1 hr and, for example, up to 6 hr, for example 30 min to 5 hr or 30 min to 4 hr; intervals of up to 2 hr, e.g. up to 1 hr, including 45 min or 30 min, should be mentioned. There may, for example, be 2, 3, 4, 6, 8 or more administrations in a divided dose; these administrations may, for example, be made over a period in the range of from 30 min to 8 hr or longer, more usually up to 6 hr, for example up to 4 hr. Administration by the divided dose schedule may, if desired, be repeated once or more, after a suitable interval; such interval may be lengthy as in conventional therapy or may be shorter as in the present invention.

Thus the active ingredient may be administered over a period in the range of from 30 min to 8 hr, more usually from 1 hr to 8 hr, or more, the period depending on the interval(s) between individual constituent doses/repeats, on the number of doses and on the duration of each administration.

Thus, DACA or other compound of the general formula

I may be administered, for example, in a divided dose over a period of up to 8 hours, for example up to 6 hours, advantageously up to 5 hours, e.g. up to 4 hours, for example 2 to 4 hours, followed by a rest, for example for 3 to 4 weeks. The dose may be divided into two to four administrations over the 2 to 4 hour or other administration period, and the first dose may be larger than the others, that is, as a loading dose; administra¬ tions may be given intravenously. For example, a short- term intravenous infusion of 10 to 30 minutes (for example 15 minutes) may be used, followed by a further such infusion after, for example, 1 hour. This schedule differs from that normally used for other cytotoxic agents, which involves periods of intravenous infusion administered daily, for example for 3 to 7 days, or long- term intravenous infusion over a number of days, for example for a week.

The first constituent dose is advantageously close to the maximum tolerable dose; that is, the maximum plasma level reached after the administration is close to the maximum that can be tolerated. Dosage schedules in which the second administration is made when or after the level of active ingredient in the blood has begun to fall, and optionally one or more further administrations are made also when or after the then level of active ingredient has begun to fall should be mentioned. Thus the rate of administration of the total dose over the

administration period is not uniform, and the split doses provide cyclic peaks/troughs in plasma levels while giving lower steady-state concentrations in tumour tissue. Dosage schedules in which each trough for the plasma level is, for example, no less than 5% of the original maximum and each peak in the plasma level is, for example, up to 120% of the original maximum should especially be mentioned. Each trough for the plasma level may, for example, be in the range of from 5 to 50% of the original maximum, and each peak for the plasma level may, for example, be in the range of from 90 to 120% of the original maximum.

The second constituent dose may, for example, be administered when the plasma level of the drug has dropped to a value in the range of from 5 to 30% of the maximum level reached after administration of the first constituent dose, and any subsequent dose may, for example, be administered when the plasma level has dropped to a value in the range of from 10 to 50% of the maximum level reached after the immediately preceding administration.

The present invention also provides a pack compris¬ ing a divided-dose medicament for the treatment of cancer, each dose comprising a compound of the general formula I or a physiologically acid addition salt or N- oxide thereof, in admixture or conjunction with a pharmaceutically suitable carrier, together with

instructions for administration of the doses at frequent intervals, for example as mentioned above.

The pack may, for example, be provided together with means for sampling blood levels or for measuring plasma level. Alternatively, administration may be made at intervals established previously by administration of the drug to other subjects.

We have found that DACA also has activity against melanoma cell lines and human tumour xenografts of these lines, and it is believed that this activity is improved by the same dosing strategy mentioned above.

The cytotoxicity of DACA was assessed in a panel of primary human melanoma cultures derived from fresh surgical melanoma specimens. IC5 ₀ values ranged from 0.2 to 1.5 μM, and a feature of the data was the ability of DACA to kill much higher proportions of cells (>99%) in some cultures, as compared to a maximum of 90% for etoposide.

A further experiment was carried out using human melanoma line, implanted subcutaneously in nude (athymic) mice. Treatment was started when the tumours were 4-7 mm in diameter. DACA was administered ip as a divided dose (2 x 100 mg/kg body weight at 0 and 60 min) and a second similar administration (2 x 100 mg/kg) was given after 7 days. A growth delay of 30 days was obtained.

The positive results achieved by DACA in these treatments is surprising since melanoma is more difficult

to treat with chemotherapy than are other forms of tumour.

Moreover, Berger et al. (Berger D.P., Winterhalter B.R. , Flebig H.H. , "Conventional chemotherapy" in "The Nude Mouse in Oncology Research", 1st ed. London: CRC Press, 1991, 165-84, ed. Boven and Winograd) states that melanoma xenografts are resistant to treatment by doxorubicin and etoposide, so activity by a drug in this class is completely unexpected. A summary of the activity of various other agents against subcutaneous melanoma xenografts growing in nude mice is given by Berger et al. as follows:

Drug Percentage of xenografts responding Topo II inhibitors Doxorubicin 5% (total of 5 studies) Etoposide 0% (2 studies)

- 34 -

Vinblastine 10%

Vincristine 43%

In pharmacokinetic studies using radioactive (tritium-labelled) DACA high levels of active ingredient have been found in all tissues, including brain, with a long elimination t _] _/ ₂ ^of 37-176 h. As determined by HPLC, the tissue concentrations of DACA 1 h after intraperitoneal administration of drug (400 μmol/kg) were 45, 185, 139, and 57 μmol/kg in brain, liver, kidney and heart, respectively. The corresponding AUC values (AUC = area under the plasma concentration-time curve) were 218, 547, 492 and 147 μmol.h/1, respectively, as compared to the plasma AUC of 26.6 μmol.h/1. DACA showed relatively high rates of passage across the blood brain barrier. We believe that administration of DACA at the new divided dose-high constituent dose frequency regime mentioned above will be helpful in combating brain tumours. With the exception of nitrosoureas, few of the antitumour agents currently in use possess the physicochemical properties required for adequate penetration of the blood-brain barrier (Greig M.H. (1987) , Cancer Treat. Rep. 11: 157) .

We also propose the use of DACA and related compounds with a "rescue" treatment with a second drug which by itself is not an active agent but which dis¬ places DACA or the other compound from the DNA. This

DNA-binder, or chemoprotector, should have a lower intrinsic toxicity and less efficient tissue distribution properties than the cytotoxic agent, thus sparing rapidly — growing and highly vascularised normal tissues such as bone marrow from cytotoxic effects. Use of the new schedule of administration of DACA or other compound of the general formula I or a physiologically tolerable acid addition salt or 1-N-oxide thereof, combined with "rescue" treatment with a chemoprotector, should especially be mentioned. Timing of administration of the chemoprotector will depend on the pharmacokinetics of DACA or other drug used. The chemoprotector may, for example, be administered at the same time as or up to 30 minutes after one or more of the constituent doses of a divided dose of that drug; by such means doses of, for example, 200 mg/kg or even 300 mg/kg of DACA - doses which are normally toxic - may be possible.

The following Examples illustrate the invention. Examples Example 1

Activity of DACA against cultured multidrug resistant human leukaemia cells Materials and Methods

Acridine carboxamide hydrochloride, synthesised in the Cancer Research Laboratory (Atwell G.J., et al., J.Med. Chem. 1987, 30, 664-669), and amsacrine

isethionate, obtained from the Parke-Davis Division of the Warner-Lambert Company, Ann Arbor, USA, were dis¬ solved in 50% v/v aqueous ethanol to make stock solutions of 2-5 mmol/1 and stored at -20°C. Other cytotoxic drugs were available either from the NCI repository (Monks A. , et al., J. Natl. Cancer Inst. 1991, 83, 757-766) or were obtained as described in Marshall E.S., et al., J. Natl. Cancer Inst. 1992, 84, 341-344 and Finlay, et al., Eur. J. Cancer Clin. Oncol. 1986, 22, 655-662. Cell lines were from the NCI repository except for MM-96 (Dr. R. Whitehead, Ludwig Institute, Melbourne, Australia) , FME (Dr. K.M. Tveit, Norwegian Radium Hospital, Oslo, Norway) and Jurkat normal and multidrug-resistant lines (Dr. K. Snow and Dr. W. Judd, Department of Cellular and Molecular Biology, University of Auckland) . Melanoma tissue was obtained from patients with pathologically confirmed metastatic and recurrent melanomas under Auckland Hospital Ethical Committee guidelines. Cells were released by digestion of tissue (at 50 mg.ml ^-1 ) with collagenase (1 mg.ml ^-1 ) and DNAase (50 μg.ml ^-1 ) with continuous stirring at 37°C for 1 to 2 hours, and cultured as previously described (Marshall E.S., et al., J. Natl. Cancer Inst. 1992, 84, 341-344).

Tumour cell lines were cultured in 96-well plates. Growth of NCI cell lines was assessed using sulpho- rhodamine B staining (Skehan P., et al., J. Natl. Cancer Inst. 1990, 82, 1107-1112), that of the leukaemia lines

with (4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium (MTT) staining (Mosmann T. , J. Immun. Methods 1983, 65, 55-63) and that of the primary human tumour material by ³ H-thymidine incorporation (Marshall E.S., et al. , J. Natl. Cancer Inst. 1992, 84, 341-344). Primary tumour material was cultured in 96-well plates in which the wells were coated with agarose to inhibit selectively the growth of normal cells. (Marshall E.S., et al., J. Natl. Cancer Inst. 1992, 84, 341-344). Primary cultures were incubated at 37°C in sealed perspex boxes containing a humidified atmosphere of 5% C0 ₂ and 5% 0 ₂ in nitrogen for 7 days. 5-Methyl-[ ³ H]-thymidine (20 Ci.mmol ^-1 ; 0.04 μCi per well), thymidine and 5-fluorodeoxyuridine (each at final concentrations of 0.5 μM) were added in medium to cultures (20 μl per well) 24 h before terminat¬ ing the cultures. Cells were aspirated on to glass fibre filters using a multiple automated sample harvester (LKB Wallac OY Beta Harvester) . The filter discs were washed for 15 seconds with water, dried, and the amount of tritium retained quantified by liquid scintillation.

IC ₅₀ values were defined as in Paull K.D., et al., J. Natl. Cancer Inst. 1989, 81:1088-1092, where growth, as indicated by staining or thymidine incorporation, cor¬ responded to 50% of that of the control cultures. DELTA values were determined for groups of logarithmic IC5 ₀ values as deviations from the mean, with positive DELTA values representing higher drug sensitivity relative to

the mean. Variances of DELTA values were expressed as standard deviations in log ₁₀ units. Comparison of DELTA values was made using Pearson correlation coefficients. Resistance factors were defined as the ratios of IC5 ₀ values between the resistant line and the parent line. Statistical evaluation was performed either with NCI programmes, with RS/1 software (BBN Research Systems, Cambridge, MA, USA) , or with Sigmaplot (Jandel Scien¬ tific, San Rafael, CA, USA). Results

The effect of DACA on the growth of cultured cells was assessed by continuous drug exposure. DACA inhibited the growth of two human Jurkat T cell leukaemia lines, one diploid (L) and the other tetraploid (Bl) , with IC5 ₀ values each of 380 nM. These values were similar to those of other human leukaemia cell lines, which ranged from 290 to 760 nM (CEM-CCRF, 410 nM; MOLT-4, 290 nM; Daudi, 400 nM; Raji, 370 nM, U937, 590 nM; HL-60, 620 nM; K-562, 760 nM) . Four multidrug-resistant cell lines, developed from JL and JB1 by in vitro exposure to increasing concentrations of doxorubicin (L^ and Blp) or amsacrine (L^ and Bl^) (Finlay G.J., et al., J. Natl. Cancer Inst. 1990, 82, 662-667, Snow K. , et al., Br. J. Cancer 1991, 63, 17-28) were tested. DACA was compared with six other drugs including four topo II poisons, doxorubicin, mitozantrone, etopo¬ side and amsacrine. Resistance factors for the topo II

poisons were consistently higher than those for DACA (Table 1) . In contrast, the topoisomerase I poison camptothecin showed no cross resistance, and the mitotic inhibitor vincristine showed a different pattern of resistance with the B1 _D line having the highest resis¬ tance (Table 1) .

Table 1. Drug-resistant Jurkat Leukaemia sublineε.

Resistance factors

doxo- mitoxan- etopo- amsa- DACA campto- vincris- rubicin trone side crine thecin tine

L _A 3.8 11 130 2.0 1.0 1.5

_Q 16 160 93 110 2.5 0.97 3.6

Bl _a 11 59 22 240 3.9 0.48 2.0

B1 _D 15 8.4 83 8.8 1.9 0.86 10

One method of providing a visual comparison of the patterns of resistance is to plot DELTA values (Paull K.D., et al., J. Natl. Cancer Inst. 1989, 81, 1088-1092) where the differences in bar lengths are used as a measure of relative resistance. Figure 1 shows a comparison of DELTA values in log mean graph format for DACA, amsacrine, etoposide, doxorubicin, vincristine, 5- fluorouracil, camptothecin and mitozantrone for the panel of multidrug resistant Jurkat leukaemia lines using MTT

staining; JL/AMSA and JB/AMSA were selected for resis¬ tance to amsacrine and JL/DOX and JB/DOX were resistant to doxorubicin. DACA clearly shows a pattern distinct from doxorubicin. A second method of comparing agents is to plot resistance factors for one of the lines against another. Since the Jurkat lines exhibited predominantly "altered topoisomerase" resistance (Finlay G.J., et al., J. Natl. Cancer Inst. 1990, 82, 662-667, Sugimoto Y., et al., Cancer Res. 1990, 50, 7962-7965) , the resistance factors for one of these (L^) was plotted versus the resistance factors for a P-glycoprotein positive multidrug resistant P388 Leukaemia line (P/DACT) which exhibits transport resistance (Baguley B.C., J. Natl. Cancer Inst. 1990, 82, 398-402) . The results (Figure 2) indicate that DACA is unique when compared to other topo II agents in that it is able to overcome two different multidrug resistance mechanisms. Qualitatively similar graphs are obtained when the resistance factors of the other resistant Jurkat lines are plotted on the abscissa, or those from a P- glycoprotein positive, vinblastine resistant human leukaemia line (CEM/VLB ₁₀ θ) (Qi ^{an χ} - ι Beck W.T., Cancer Res. 1990, 50, 1132-1137) are plotted on the ordinate. DACA was also compared with three other topo II agents using a panel of cell lines (data provided by Dr. Ken Paull from the National Cancer Institute, USA) encompassing a number of tumour types, and using protein

staining. The mean IC ₅₀ for DACA was 2,100 nM, as compared with amsacrine (520 nM) , etoposide (21,000 nM) and doxorubicin (140 nM) . The results, presented as DELTA plots, are compared with corresponding plots for three other topoisomerase II poisons in Figure 3. The variance of DELTA values was considerably smaller for DACA (0.24 units) than it was for amsacrine (0.61 units) etoposide (0.55 units) or doxorubicin (0.44 units) . The differences in DELTA values for amsacrine, etoposide and doxorubicin for primary human cultures imply that intrinsic resistance mechanisms exist and are partially overcome with DACA.

DACA was also compared in a series of 12 primary melanoma cultures. Tissue was excised from human malignant melanomas and cultured using a modified 96-well assay system in which the cells were cultured on agarose and assayed for proliferation using the ³ H-thymidine incorporation assay as described in Marshall E.S., et al., J. Natl. Cancer Inst. 1992, 84, 341-344. The mean IC ₅₀ for DACA was 590 nM, as compared with amsacrine

(128 nM) , etoposide (2,200 nM) and doxorubicin (56 nM) . DELTA values for DACA, amsacrine, etoposide and doxo¬ rubicin are compared in Figure 4. The variance of DELTA values was smaller for DACA (0.39 units) than for amsacrine (0.54 units), etoposide (0.66 units) or doxorubicin (0.63 units). The differences in DELTA values for amsacrine, etoposide and doxorubicin for

primary human cultures again imply that intrinsic resistance mechanisms exist and are partially overcome with DACA.

Example 2 Activity of DACA against advanced colon 38 and melanoma in mice Materials and Methods

Colon 38 carcinoma was obtained from the Mason Research Institute (Worchester, MA, USA) and was grown in BDF _! hosts. Tumour fragments (1 mm ³ ) were implanted subcutaneously in anaesthetised mice. Tumours had grown to the appropriate size 9 days after implantation. A melanoma tumour line (WADH) was developed in the Cancer Research Laboratory. Tumour cells were grown in culture and 1 x 10 ⁶ cells were implanted intradermally into the flank of nude (C57BI/J genetic background) mice. Mice were grown under sterile surroundings until tumours were of appropriate size.

Tumours were measured 3x (colon 38) or 2x (xeno- graft) weekly with callipers and tumour volumes calculated as 0.52a ² b, where a and b were the minor and major tumour axes. Tumour growth delays were measured at a time when tumour volumes of treated and control animals had increased by 4-fold. Results

The effects of five compounds,

( i) DACA

(ii) N-[2-(dimethylamino)ethyl]-9-methoxyphen- azine-1-carboxamide (compound 28 of J. Med. Chem., 1987, 30, 843-851), (ϋi) N-[2-(dimethylamino)ethyl]-2-phenylquino- line-8-carboxamide (compound 6 of J. Med. Chem. 1988, 31, 1048-1052), (iv) N-[2-(dimethylamino)ethyl]-2-(4-pyridyl)- quinoline-8-carboxamide (compound 18 of J. Med. Chem., 1989, 32, 396-401), and

(v) N-[2-(dimethylamino)ethyl]-9-bromo-dibenzo- [l,4]-dioxin-l-carboxamide (compound 20 of J. Med. Chem. 1992, 35, 258-266) on the growth of advanced colon 38 tumours in mice was investigated by implanting tumour fragments subcu¬ taneously and allowing them to grow until they had reached a diameter of 5-8 mm.

I.p. treatment of mice with a single maximum tolerated dose of DACA (150 mg/kg body weight) , a treatment which was known to induce cures of intra¬ venously implanted Lewis lung tumours (Finlay G.J., Baguley B.C., Eur. J. Cancer Clin. Oncol. 1989, 25, 271- 277) caused only a slight growth delay (5 days; Figure 5) . However, when a divided dose (200 mg/kg) was administered over a period of 0.5 - 4 hours, greater delays were unexpectedly observed (Table 2) . Repetition of these divided doses provided a substantial growth

delay (23 days; Figure 5) which was longer than that obtained with the maximum tolerated dose of amsacrine (2 days), cyclophosphamide (6.5 days) or 5-fluorouracil (13 days) .

Table 2. Tumour growth delays (colon 38) treated with DACA

Total dose Schedule Growth delay (days)

Note: the 4 dose schedule was 65 + 45 + 45 + 45 mg/kg

The other compounds were each administered by ip injection in two doses one hour apart at a dose (for each injection) of lOOmg/kg for the phenazine, 75 mg/kg for the phenylqumoline, 50mg/kg for the pyridylquinoline and 100 mg/kg for the dibenzodioxin. The growth delays obtained are shown in Table 3, together with the result obtained with DACA at the same schedule of administration.

Table 3. Tumour growth delays (colon 38) treated with tricyclic carboxamide derivatives.

Compound Dose Route Schedule Growth delay each (days) injection

10,12 (2 expts)

5,5 (2 expts)

8.5

A further experiment was carried out using human melanoma line, implanted subcutaneously in nude (athymic) mice using an inoculation of one million cells of a human melanoma cell line designated WADH. Treatment was started when the tumours were 4-7 mm in diameter. DACA was administered ip as a divided dose (2 x 100 mg/kg body weight at 0 and 60 min) and a second similar administra- tion (2 x 100 mg/kg) was given after 7 days. A growth delay of 30 days was obtained (Figure 6) .

Example 3

Exploitation of the self-inhibitory properties of a drug in the therapy of solid tumours

One of the characteristics of solid tumours is that because of the poor vascularisation, oxygen, nutrients and chemotherapeutic drugs must diffuse for longer distances than they do in normal tissue (Wilson W.R. , Denny W.A. , Radiation Research: a Twentieth Century Perspective, 1st ed. v. 2. New York: Academic Press, 1992:796-801). In the case of antitumour agents, a gradient of drug concentration is established with the lowest drug concentration at greatest distances from the capillary. Since in all cases examined so far with existing clinical agents, cytotoxicity is related in a positive fashion to drug concentration, it follows that those areas most remote from the tumour blood supply are protected from drug cytotoxicity, a so-called "pharma¬ cological sanctuary".

DACA is a DNA intercalating agent which acts on topo II and has the unusual property of inhibiting its own toxicity at concentrations above 5 μm. It also inhibits the formation of DNA-protein cross-links above 5 μM, consistent with the hypothesis that self-inhibition of DNA-protein cross-links is related to self-inhibition of toxicity. A simple model for this behaviour is that in order for topo II to form its complex with DNA (i.e. to form DNA-protein cross-links) it requires the presence of

a DNA-drug complex (probability = p) , surrounded on each side by drug-free DNA (probability = (1-p)). It follows that the probability of forming a productive complex is p(l-p) ² . When this function is plotted against experi- mental cytotoxicity data for DACA (Haldane A. , et al. , Cancer Chemother. Pharmacol. 1992, 29, 475-479), a good approximation is obtained (Figure 7) .

Figure 7 can also be plotted as toxicity versus cell-associated drug (using unpublished data from the Cancer Research Laboratory which relates external drug concentration to cell-associated drug) . It can be seen from Figure 7 that if a tumour concentration gradient is established whereby the area of the tumour closest to the capillary has, for example, a concentration of 1800 μmol/kg, areas of the tumour which are more remote from the capillary, although having a lower drug concen¬ tration, will have higher cytotoxicity. Furthermore, host tissues, which have good blood supplies, will have high tissue drug concentrations and thus lower cyto- toxicity. By this principle, DACA (and other compounds of this general class) could have a selectivity mechanism for solid tumours which is not possessed by other agents.

The practical application of this hypothetical situation requires that free drug plasma concentrations (and corresponding tissue concentrations of drug) fall into the range which will provide selectivity (i.e. greater than 1000 μmol/kg tissue) . Some findings

indicate that when DACA is administered at a maximally tolerated single drug dose (150 mg/kg body weight) , drug concentrations in normal tissues (e.g. liver, spleen) slightly exceed 1000 μmol/kg. This principle may be exploited further by drug design or by combining DACA administration with that of a second chemoprotector agent which increases the self-inhibition of DACA (i.e. the descending part of the curve in Figure 7) and thus lowers the average tissue drug concentration required for the application of this principle.