WATER-SOLUBLE ETHYLIDENE PHOSPHATE PRODRUG OF PROPOFOL

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a novel water-soluble prodrug of propofol, its pharmaceutically acceptable salts, methods of preparing the prodrug, pharmaceutical compositions containing the prodrug, and methods of using the prodrug. In particular, the present invention relates to a novel prodrug of propofol which is less toxic than phosphonooxymethyl ether prodrug of propofol (Aquavan®) undergoing clinical trials.

BACKGROUND OF THE INVENTION

Solubility limitations of drug molecules are an emerging challenge for the pharmaceutical industry with an objective to find pharmacologically more and more potent drug molecules. Since a sufficient aqueous solubility is crucial in both i.v. and non- parenteral administration and because the possibilities to formulate the poorly water- soluble drug molecule are limited, the prodrug technology has become a widespread method in enhancing the aqueous solubility of drug molecules.

Phosphate prodrugs offer several advantages for formulation and development of poorly water-soluble compounds. Phosphates have been synthesized to hydroxyl or amine functionalities of a parent drug either directly or via an oxymethyl spacer group. Oxymethyl spacers are generally used to increase the space around the enzy- matically cleavable bond (phosphates, esters, carbonates) and they undergo fast spontaneous chemical hydrolysis after enzymatic hydrolysis of the promoiety (Safadi et al, Pharm Res 1993, 10, 1350-1355; Varia et al, J Pharm Sci 1984, 73, 1074- 1080). The possible drawback of the oxymethyl linker structure is the systemic Hb- eration of toxic formaldehyde in the body, which has effects in altered homeostasis in cells (such as induction, metabolic switch and cell proliferation) (Heck et al., Crit

Rev Toxicol 1990, 20, 397-426). An alternative strategy for the sufficiently fast enzymatic hydrolysis rate and preferable safety profile of prodrugs is an ethyloxy- spacer structure (further called as ethylidene structure) that liberates much less toxic acetaldehyde from the prodrug structure.

Ethylidene structure has not earlier been reported for phosphate prodrugs but has been widely used to link an ester, a carbonate or a carbamate promoiety to parent drugs containing carboxylic acid functionality. These prodrugs include such as can- desartan cilexitil (Gleiter and Morike, Clin Pharmacokinet 2002, 41, 7-17), bacam- picillin (Bergan, Antimicrob Agents Chemother 1978, 13, 971-974), cefpodoxime proxetil (Klepser et al, Clin Pharmacokinet 1995, 28, 361-384), cefuroxime axetil (Williams and Harding, J Antimicrob Chemother 1984, 13, 191-196), and bacmecil- linam (Josefsson et al., Eur J Clin Pharmacol 1982, 23, 249-252).

Propofol (2,6-diisopropylphenol; Formula 1) is a low molecular weight phenol that is widely used anesthetic with rapid onset, short duration of action and minimal side effects (Baker et al., Anesthesiology 2005, 103, 860-876). Propofol has also a wide range of other biological and medical applications. Propofol has been shown to treat and/or prevent migraine headache pain (Krusz et al., Headache 2000, 40, 224-230), reduce a post-operative nausea and vomiting (Tramer et al., Br J Anaesth 1997, 78, 247-255; Brooker et al., Anaesth Intensive Care 1998, 26, 625-629), possess antiemetic activity (Phelps et al., Ann Pharmacother 1996, 30, 290-292, Tomioka et al., Anesth Analg 1999, 89, 798-799), and treat refractory status epilepticus (Kuisma et al., Epilepsia 1995, 36, 1241-1243; Walder et al., Neurology 2002, 58, 1327- 1332). Moreover, the anticonvulsant effects of propofol have been shown in rat efficacy models (Holtkamp et al., Ann Neurol 2001, 49, 260-263). Propofol has also been used as an antioxidant (Wang et al., Eur J Pharmacol 2002, 452, 303-308; De La Cruz et al., Anesth Analg 1999, 89, 1050-1055).

The extremely low water-solubility, high lipophilicity, inherent emulsion instability, pain on injection and hyperlipidemia on prolonged administration impede the use of

propofol. For example, in a current commercial product (Diprivan ^® , Astra-Zeneca) an oil-in-water emulsion (the emulsifϊer is the lecithin mixture Intralipid ^® ) is used to formulate propofol (Picard et al, Anest. Analg. 2000, 90, 963-969).

Propofol (1 )

One possible solution to the insolubility of propofol in aqueous solution, which avoids the use of additives, solubilizers or emulsifϊers, is a water-soluble propofol prodrug that is bioconverted to propofol in vivo. Prodrugs of propofol have been reported (e.g., Gallop et al., WO 2005/023204; Gibiansky et al., WO2007008869; Rogers et al., UA 76 802 C2; Wozniak et al., CA 2 548 216 Al). Moreover, phosphate prodrugs of propofol (GPI 15715, Aquavan ^® ), but not phosphonooxyethyloxy ether of propofol, have been described in the literature (Baker et al., Anesthesiology 2005, 103, 860-876; Gibiansky et al., Anesthesiology 2005, 103, 718-729; Struys et al., Anesthesiology 2005, 103, 730-743; Schywalsky et al., Eur J Anaesthesiol 2003, 20, 182-190; Stella et al., International Publication No. WO 00/08033). The previously known phosphonooxymethyl ether of propofol has an oxymethyl spacer between propofol and the phosphate promoiety. It enhances the water-solubility of propofol by many-fold, but liberates toxic formaldehyde as a breakdown product in the body. Thus, there is a need for propofol prodrugs, which are readily water- soluble but do not release toxic byproducts during bioconversion in vivo.

BRIEF DESCRIPTION OF THE FIGURE

Figure 1 shows propofol levels after single i.v. bolus dose of A) propofol phosphate according to the invention (19.4 mg/kg) and B) propofol (10 mg/kg) into rats (n = 3). Each line shows the measured values of one rat.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concerns the novel bioreversible ethyl dioxy phosphate of propofol of the formula

Ethyl dioxy phosphate of propofol wherein:

Z represents hydrogen, an alkali metal ion, an alkaline earth ion, an aluminium ion; ethanolamine, diethanolamine, triethanolamine, or N-methylglucamine, and pharmaceutically acceptable salts thereof.

When Z is hydrogen, the pharmaceutically acceptable salts of the novel ethyl dioxy phosphate of propofol include acid addition salts, formed with inorganic acids such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid, and the like; or formed with organic acids such as acetic acid, propionic acid, hexa- noid acid, cyclopentanepropionic acid, glycolic acid, puryvic acid, lactic acid, malo- nic acid, succinic acid, malic acid, maleic acid, fumaric acid, tartaric acid, citric acid, benzoic acid, 3-(4-hydroxybenzoyl) benzoic acid, cinnamic acid, mandelic acid, methanesulfonic acid, ethanesulfonic acid, 1,2-ethane-disulfonic acid, 2- hydroxyethanesulfonic acid, benzenesulfonic acid, 4-chlorobenzenesulfonic acid, 2- naphthalenesulfonic acid, 4-toluenesulfonic acid, camphorsulfonic acid, A- methylbicyclo[2.2.2]-oct-2-ene-l-carboxylic acid, glucoheptonic acid, 3- phenylpropionic acid, trimethylacetic acid, tertiary butylacetic acid, lauryl sulfuric

acid, gluconic acid, glutamic acid, hydroxynaphthoic acid, salicylic acid, stearic acid, muconic acid, and the like.

In the following description "phosphate prodrug" refers to the ethyl dioxy phosphate of propofol according to the invention.

Phosphate prodrug and/or pharmaceutical compositions thereof are preferably administered intravenously. Phosphate prodrug and/or pharmaceutical compositions thereof may also be administered by any other convenient route, for example, orally, by absorption through epithelial or mucocutaneous linings (e.g., oral mucosa, rectal and intestinal mucosa). Administration can be systemic or local. Various delivery systems can be used for administration (e.g., encapsulation in liposomes, microparti- cles, microcapsules, capsules). Methods for administration include, but are not limited to, intradermal, intramuscular, intraperitoneal, intravenous, subcutaneous, intra- nasal, epidural, oral, sublingual, intracerebral, intravaginal, transdermal, rectal, topical (e.g., skin, eyes, nose, ears), or by inhalation.

A pharmaceutical composition according to the invention comprises the phosphate prodrug of the invention and a pharmaceutical acceptable vehicle, with which the compound is administered to a patient. Water is a preferred vehicle when the compound is administered intravenously. Saline solutions and aqueous dextrose and glycerol solutions can also be employed as liquid vehicles, particularly for injectable solutions. Suitable pharmaceutical vehicles also include excipients such as starch, glucose, lactose, sucrose, gelatin, malt, rice, flour, chalk, silica gel, sodium stearate, glycerol monostearate, talc, sodium chloride, dried skim milk, glycerol, propylene, glycol, ethanol, and the like. The present pharmaceutical compositions, if needed, can also contain minor amounts of wetting or emulsifying agents, or pH buffering agents. In addition, auxiliary, stabilizing, thickening, lubricating and coloring agents may be used.

The compositions according to the invention can take the form of solutions, suspensions, emulsion, tablets, pills, pellets, capsules, capsules containing liquid, powders, sustained-release formulations, suppositories, emulsions, aerosols, sprays, suspensions, or any other form suitable for pharmaceutical use.

The invention is also directed to a process for preparing the novel ethyl dioxy phosphate of propofol, the method comprising the steps of synthesizing a vinyl ether of propofol, followed by the addition reaction of HCl gas to vinyl ether to yield 1- chloroethyl ether, further derivatization to phosphate by using tetrabutylammonium phosphate in acetonitrile with an excess of triethylamine and, optionally, the treatment of the product with an appropriate base to obtain pharmaceutically acceptable salts.

A further object of the invention is the use of the novel ethyl dioxy phosphate of propofol for the preparation of a medicament e.g. for providing anesthesia, for treating or preventing migraine headache, for reducing post-operative nausea and vomiting, for providing anti-emetic activity, for treating refractory status epilepticus or for use as an anticonvulsant.

Another object of the invention is a method for increasing aqueous solubility, dissolution and bioavailability of propofol, comprising the step of preparing a propofol prodrug of the formula

Ethyl dioxy phosphate of propofol

wherein Z is as defined above, or a pharmaceutically acceptable salt thereof.

A further object of the invention is a method of treatment which produces an anes- thetic effect in a mammal, comprising administering to said mammal an effective amount of a prodrug compound according to the invention. The prodrug compound according to the invention can also be used in a method for treating or preventing migraine headache, for reducing post-operative nausea and vomiting, for providing anti-emetic activity, for treating refractory status epilepticus or for use as an anti- convulsant in a mammal, which method comprises administering to said mammal an effective amount of a prodrug compound according to the invention.

The following examples illustrate the invention without limiting the same in any way.

EXAMPLES

Synthesis of the ethyl dioxy phosphate of propofol

Chemistry. All the described reactions were monitored by thin-layer chromatogra- phy using aluminum sheets precoated with Merck silica gel 60 F254. Samples were visualized by UV-light. Column chromatography was executed on Merck silica gel

60 F ₂₅₄ (0.063-0.200 mm mesh). ¹ H, ¹³ C and ³¹ P-NMR-spectra were recorded on a

Bruker Avance DRX500 spectrometer (Bruker, Rheinstetter, Germany) operating at

500 MHz, 125.76 and 202.46 MHz at 25 ⁰ C, respectively. TMS for CDCl ₃ -samples and TSP for D ₂ O samples were used as an internal reference. The splitting pattern abbreviations are as follows: s = singlet, d = doublet, t = triplet, q = quartet, h = heptet, m = multiplet. Electronspray ionization mass spectra were acquired by an

LCQ quadrupole ion trap mass spectrometer with an electronspray ionization source

(Finnigan MAT, San Jose, CA). Elemental analyses were carried out on a Thermo- Quest CE Instruments EA 1110-CHNS-O elemental analyzer. All reagents and en-

zymes were obtained from commercial suppliers and were used without further purifications.

2-(l-Chloro-ethoxy)-l,3-diisopropyl-benzene (2). Ethyl acetate was saturated with dry HCl-gas by bubbling gaseous HCl through the solution for 10 min. 2 (1.04 g, 5.1 mmol) was dissolved in 10 mL of HCl-saturated ethyl acetate. The mixture was heated in microwave-oven at 100 ⁰ C at 5 bars for 10 min and evaporated to dryness. The next reaction was continued immediately without any further purification. Phosphoric acid mono-[l-(2,6-diisopropyl-phenoxy)-ethyl] ester disodium salt (3). To a solution of tetrabutylammonium phosphate (0.4 M in acetonitrile, 30 mL, 12 mmol) was added 2 (5.1 mmol, theoretical maximum yield) in triethylamine (5 mL, 36 mmol) under argon. The reaction mixture was stirred for 18 hours and the solvents were evaporated. 30 mL of water was added and the residue was extracted with diethylether (3 x 50 mL). Solvents were evaporated, 10 mL of acetonitrile was added and the pH of the mixture was adjusted to 11 with saturated NaOH. After stirring of 10 minutes the solvents were evaporated, the residue was purified with preparative HPLC on reversed phase Kromasil 100 A (C8) column by gradient elu- tion using water and acetonitrile (40-80 % ACN) as eluent and lyophilized to obtain 3 as white solid (494 mg, 1,43 mmol, 28 %). ¹ H NMR (D ₂ O) δ 7.29-7.22 (3H, M), 5.54 (IH, q, J = 5.1 Hz), 3.42 (2H, h, J = 6.9 Hz), 1.66 (3H, d, J = 5.1 Hz), 1.21 (12 H, t, J = 6.9 Hz). ¹³ C NMR (D ₂ O) δ 152.30, 146.45, 128.57, 127.08, 103.04, 29.28, 26.03 (d, J = 15.2 Hz), 24.97. ³¹ P NMR (D ₂ O) δ 0.03. ESI-MS: 301.3 (M - 2*Na + 1). Anal. Calcd for Ci ₄ H ₂ iNa ₂ 0 ₅ P ^• 0.7 H ₂ O: C, 46.85; H, 6.29. Found: C, 46.79; H, 6.29.

Evaluation of the ethyl dioxy phosphate of propofol

The novel water-soluble prodrug of propofol has been evaluated in in vitro and in vivo studies. Aqueous solubility and conversion of the prodrug is described below.

1) Analytical method. HPLC analysis of all compounds was performed using an Agilent Technologies 1100 series gradient RP-HPLC system with UV detection (220 nm) and for rat whole blood samples the fluorescence detection (Ex = 276 nm, Em = 310). The HPLC system consisted of Agilent 1100 Series Binary Pump, 1100 Series Autosampler, 1100 Series Micro Vacuum Degasser, 1100 Series Thermo- stated Column Compartment, 1100 Series Fluorescence Detector, HP 1050 Variable Wavelenght Detector, 1100 Series Control Module and a Zorbax Eclipse XDB-C8 (4.6 mm x 150 mm, 5 μm) analytical column (Agilent Technologies Inc., Little Falls, Wilmington, DE). Loop injection volume was 20 μl. The isocratic elution was per- formed by using a mobile phase consisting of 90 % (v/v) acetonitrile and 10 mM tetrabutylammonium dihydrogenphosphate with a ratio of 65:35 at a flow rate of 1.0 mL/min at 3O ⁰ C.

2) Aqueous solubility

Method

The aqueous solubility of the propofol prodrug was determined at room temperature in Tris-HCl-buffer (50 mM) at pH 7.4. The pH of the mixtures was held constant during the study. 6 mg of propofol prodrug was added to 0.5 mL of buffer, the mix- ture was stirred for 1 hour, filtered (0.45 μm Millipore) and analyzed by HPLC.

Results

The aqueous solubility of propofol (130 ± 2 μg/mL) was greatly enhanced by making the propofol prodrug that has the solubility over 10 mg/mL. The maximum solu- bility of 3 was not determined due to the small amount of prodrug available. However, the limit was chosen since it is determined that drug should have solubility of at least 10 mg/mL to be suitable for i.v. administration.

3) Hydrolysis in aqueous solutions

Method

The rate of the chemical hydrolysis of propofol prodrug was determined at 37 ⁰ C or at room temperature in borate buffer (180 mM) at pH 7.4. An appropriate amount of propofol prodrug (concentration approx. 50 μM) was dissolved in preheated buffer solution or in buffer solution at room temperature and the solution was placed in a thermostatically controlled water bath at 37 ⁰ C or to magnetic stirrer at r.t. Samples were taken at appropriate intervals and analyzed by HPLC. Pseudo-first-order half-lives (ti/ ₂ ) for the hydrolysis of prodrugs were calculated from the slope of the linear portion of the plotted logarithm of remaining prodrug versus time.

Results

Chemical degradation of propofol prodrug followed first-order kinetics and the half- life (ti _/2 ) = 5.2 ± 0.2 days at r.t. and (t _1/2 ) = 21.5 ± 0.8 h at 37 ⁰ C.

4) Enzymatic hydrolysis in alkaline phosphatase solution

Method

Hydrolysis of propofol prodrug in alkaline phosphatase solution was determined at 37°C. To a solution of propofol prodrug in 5OmM Tris-HCl buffer (pH 7.4, final concentration 50 μM) at 37°C was added 4 μl (81.8 units) of alkaline phosphatase. In blank solutions, alkaline phosphatase was replaced with the same volume of water to ensure that the hydrolysis is particularly enzymatic. At predetermined time intervals, 200 μl samples were removed and 200 μl of ice-cold acetonitrile was added to each sample to stop the enzymatic hydrolysis. The samples were kept on ice, centri- fuged for 10 min at 14000 rpm, and the supernatant was analyzed by the HPLC. Pseudo-first-order half-life (ti/ ₂ ) for the hydrolysis of propofol prodrug was calculated from the slope of the linear portion of the plotted logarithm of remaining prodrug versus time.

Results

The enzymatic hydrolysis of propofol prodrug was determined in alkaline phosphatase solution. The enzymatic hydrolysis of propofol prodrug to propofol was extremely rapid with the half-life of approximately 20 seconds. This fast enzymatic hydrolysis of propofol prodrug suggests that ethylidene linked prodrugs act by similar manner as conventional phosphate and phosphonooxymethyl prodrugs and release the parent drug rapidly by alkaline phosphatase.

5) Bioconversion in vivo The conversion of propofol prodrug to propofol was demonstrated in vivo in rats after a single i.v. bolus administration. One group of three adult male Wistar rats received the single bolus dose of 10 mg/kg of propofol (0.056 mmol/kg) and another group of three rats received 19.4 mg/kg of propofol prodrug (0.056 mmol/kg). The plasma concentration of propofol was determined from whole blood samples using thymol as an internal standard. Only the amount of propofol, not the amount of prodrug, was determined and compared between the groups. The concentration curves of propofol after administration of propofol prodrug and propofol are presented in Figure 1 , in which each line shows the measured values of one rat. Pharmacokinetics of propofol with fast initial decline and slower elimination was best described by two-compartmental model.

The levels of propofol in whole blood (μg/ml) and the pharmacokinetic parameters (T _max , Cmax) were determined using the above mentioned simulation and are shown in Table 1.

Table 1. Estimated pharmacokinetic parameters (mean ± SD; n = 3) of propofol after single i.v. bolus dose of ethyl dioxy phosphate of propofol (19.4 mg/kg, 0.056 mmol/kg) and propofol (10 mg/kg, 0.056 mmol/kg) into rats.

Ethyl dioxy phosphate of Propofol propofol

Dose 19.4 mg /kg 10 mg / kg

(0.056 mmol/kg) (0.056 mmol/kg)

C _max (μg/mL) 3.0 ± 0.2 14.3 ± 1.8

C _max = maximum concentration of propofol T _max = time to maximum concentration of propofol c After i.v. bolus Tmax = 0 min

Propofol prodrug was readily converted to propofol in each rat after i.v. administration. Rats that received propofol prodrug were dopey and had difficulties to keep their balance. In contrast, the dose of 10 mg/kg of propofol was enough for the sedation after approximately 13 seconds of the i.v. bolus. The difference between the concentrations of propofol after equimolar administration of propofol prodrug and propofol is probably due to formulation-dependent pharmacokinetics of propofol. A lipid-free formulation of propofol has the greater volume of distribution and the greater elimination clearance, but the similar terminal half- life when compared to oil- in-water formulation (Dutta et al, JPharm Pharmacol 1998, 50, 37-42).

6) Conclusion

The results show that the novel ethyl dioxy phosphate of propofol disclosed in the present invention

1) is readily water-soluble,

2) has reasonable chemical stability in aqueous solutions,

3) is readily bioconverted to propofol by alkaline phosphatases in vitro,

4) is readily bioconverted to propofol in vivo in rats, 5) does not liberate formaldehyde from the structure.