This patent application claims the priority benefit of European Patent Application EP1 1004055.7 filed May 17, 201 1 which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to cells and methods for detecting nucleoside reverse transcriptase inhibitors and for predicting the sensitivity of a reverse transcriptase to a nucleoside reverse transcriptase inhibitor (NRTI) treatment.

BACKGROUND OF THE INVENTION

Some viral infections constitute real health problems for the society. Among them, HIV {Human Immunodeficiency Virus) infection is one of the most serious diseases affecting humankind. The development of new therapeutic strategies, such as combination of chemotherapy, has really improved the quality and the expectancy of life of patients infected by HIV. There are different classes of anti-HIV drugs, including anti- reverse transcriptase compounds, anti-integrase compounds, anti-entry compounds and anti- protease compounds. The reverse transcriptase inhibitors include nucleoside reverse transcriptase inhibitors (NRTI) and non nucleoside reverse transcriptase inhibitors (NNRTI).

Unfortunately, in response to these drugs, the virus mutes, and drug resistances appear, leading to treatment failure. Thus, viral drug resistance and sensitivity are important subject matters for scientists working on the improvement of therapeutic strategies and there is an important need for studying such mechanisms, for identifying new antiviral drugs and for managing antiviral treatments.

The reverse transcriptase of HIV is subjected to mutations, leading to resistances against reverse transcriptase inhibitors. Several tests have been developed for detecting new reverse transcriptase inhibitors. The US patents 5,714,313 and 5,462,873 describe a simple and non expensive test for detecting reverse transcriptase inhibitors in a yeast cell. However, and even if this method can be used for non nucleoside reverse transcriptase inhibitors (NNRTI), such a test is not adapted for detection of nucleoside reverse transcriptase inhibitors (NRTI). Indeed, NRTI are prodrugs which have to enter into the cell through nucleoside transporters and which have to be tri-phosphorylated by cellular kinases to be efficient; the yeast genome is devoid of the necessary genetic information for the transporters and kinases involved in the production of mono- phosphorylated nucleosides. Thus, there is a need to adapt the test described in the US patents 5,714,313 and

5,462,873, in order to test NRTI or candidate NRTI compounds.

SUMMARY OF THE INVENTION

The inventors have now developed a transformed yeast cell, wherein the specific expression of deoxycytidine kinase (dCK) and of at least one nucleoside transporter enables the nucleoside reverse transcriptase inhibitors (NRTI) to be functional into said transformed yeast cell.

The invention relates to a transformed yeast cell comprising:

- a nucleic acid sequence coding for a reverse transcriptase,

- a reverse transcription indicator,

- a nucleic acid sequence coding for a deoxycytidine kinase (dCK), and

- at least one nucleic acid sequence coding for a nucleoside transporter. Said transformed yeast cell enables to test the activity of nucleoside reverse transcriptase inhibitors (NRTI) on said reverse transcriptase.

The invention also relates to a method for screening a compound with the ability to inhibit reverse transcription comprising the steps of: i. contacting a transformed yeast cell containing a reverse transcriptase indicator according to the invention with a compound,

ii. culturing said transformed yeast cell in a selective medium, iii. detecting inhibition of growth of the transformed yeast cell compared to said transformed yeast cell that is not contacted with said compound, and

iv. selecting a compound with the ability to inhibit growth of the transformed yeast cell as being a compound that inhibit reverse transcription.

The invention also relates to a method for predicting the sensitivity of a reverse transcriptase to a NRTI compound, said method comprising the steps of:

i. producing a transformed yeast cell as previously described, comprising the reverse transcriptase to be studied, ii. contacting said transformed yeast cell with said NRTI compound,

iii. culturing the transformed yeast cell in a selective medium, iv. determining the growth of the transformed yeast cell with or without NRTI compound, and

v. deducing therefrom if the reverse transcriptase to be studied is sensitive or resistant to said NRTI treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.

Figure 1 : Reverse transcriptase (RT) activity and inhibitor susceptibility in strain carrying dCK in its genome and OATl in a piasmid. The figure 1 shows that when the reverse transcriptase from HIV-1 is expressed as described in US patent 5,714,313, in yeast Strain A (Mat a, URA-, HIS-, LEU-) (thus not carrying dCK and OAT1) , incubation for 72 hours in the presence of either Zalcitabine (ddC) (Strain A + ddc ΙΟΟμΜ), Didanosine (ddl) (Strain A + ddl ΙΟΟμΜ), Zalcitabine (AZT) (Strain A + AZT 100 μΜ) or Stavudine (d4T) (Strain A + d4T 100 μΜ), can not inhibit cell growth, then reverse transcription, measured by absorbance at 600 nm (OD @ 600 nm). When the reverse transcriptase from HIV-1 is expressed as described in US patent 5,714,313, in genetically modified Strain A that harbours a plasmid containing the nucleic acid sequence coding for the OAT1 transporter and where the dCK nucleic acid coding sequence was integrated in the yeast genome (Strain A#), incubation for 72 hours in the presence of either ddC (Strain A# + ddc ΙΟΟμΜ), ddl (Strain A# + ddl ΙΟΟμΜ), AZT (Strain A# + AZT 100 μΜ) or d4T (Strain A# + d4T 100 μΜ), inhibits cell growth, and then reverse transcription, measured by absorbance at 600 nm (OD @ 600 nm).

Figure 2: RT activity and NRTI susceptibility in strain carrying dCK and OAT1 in its genome. The figure 2 shows that when the reverse transcriptase from HIV- 1 is expressed, as described in US patent 5,714,313, in genetically modified Strain B {Mat a, URA+, HIS-, LEU+) where both nucleic acid sequences coding for the dCK enzyme and OAT1 were integrated into the yeast genome (Strain B## : Mat a, URA-, HIS-, LEU-, dCK+, OAT1+), incubation for 72 hours in the presence of either ddC (ddc ΙΟΟμΜ), ddl (ddl ΙΟΟμΜ), AZT (AZT 100 μΜ) or d4T (d4T 100 μΜ), inhibits cell growth (% inhibition), and then reverse transcription, measured by absorbance at 600 nm.

Figure 3: IC ₅ 0 determination of NRTIs in yeast strain carrying dCK and OAT1 in its genome. Reverse transcriptase activity from HIV-1 was expressed and tested, in Strain B## {Mat a, URA-, HIS-, LEU-, dCK+, OAT1+) as described in US patent 5,714,313, and incubated for 72 hours in the presence of different concentrations of either ddC, ddl, AZT, d4T or TFV. Inhibition of cell growth, then reverse transcription, was measured by absorbance at 600 nm (OD @ 600 nm), and IC50 values were defined, for each inhibitor, as the inhibitor concentration inducing growth inhibition to 50%. DETAILED DESCRIPTION OF THE INVENTION

Thus, the inventors have developed and tested new constructions and yeast cells useful for detecting and testing NRTI compounds in system derived from the method described in the US patents 5,714,313 and 5,462,873, which are incorporated herein by reference.

Yeast cells of the invention

A first object of the invention relates to a transformed yeast cell comprising:

- a nucleic acid sequence coding for a reverse transcriptase,

- a reverse transcription indicator,

- a nucleic acid sequence coding for a deoxycytidine kinase (dCK), and

- at least one nucleic acid sequence coding for a nucleoside transporter. These elements may be comprised in the genome or in plasmids.

In a preferred embodiment, the invention relates to a transformed yeast cell comprising:

- a nucleic acid sequence coding for a reverse transcriptase and a reverse transcription indicator in a plasmid, and

- a nucleic acid sequence coding for a deoxycytidine kinase (dCK) and at least one nucleic acid sequence coding for a nucleoside transporter in its genome.

Said transformed yeast cell enables to test the activity of nucleoside reverse transcriptase inhibitors (NRTI) on said reverse transcriptase.

The term "NRTI" or "nucleoside reverse transcriptase inhibitor" refers to a nucleoside analog used as an antiretroviral drug whose chemical structure constitutes a modified version of a natural nucleoside. Such compounds suppress replication of retroviruses by interfering with the retroviral reverse transcriptase by taking the place of physiological nucleosides in the viral retro transcription event leading to random arrest of the viral nascent DNA. They can also be used for inhibiting all form of reverse transcriptase, particularly non retroviral reverse transcriptase. The NRTI group encompasses, but is not limited to, Zidovudine (also called AZT, RETROVIR), Didanosine (also called ddl, VIDEX), Zalcitabine (also called ddC, HIVID), Stavudine (also called d4T, ZERIT), Lamivudine (also called 3TC, EPIVIR), Abacavir (also called ABC, ZIAGEN), Emtricitabine (also called FTC, EMTRIVA), Tenofovir (also called TFV VIREAD), Entecavir (BARACLUDE), Apricitabine (phase III clinical trial).

By "transformed yeast cell" is meant a yeast cell into which a vector (or a DNA fragment) of interest is transferred by any means, such as by infection, conjugation, transformation, electroporation, microinjection,. Methods of cell transformation are well known in the art.

In one embodiment, the transformed yeast cell of the invention is a Saccharomyces cerevisiae transformed cell. As used herein, a "reverse transcriptase" refers to a protein which has several activities, including a RNA-dependent DNA polymerase activity, and/or a RNase H activity and/or a DNA-dependent DNA polymerase activity; preferably at least a RNA-dependent DNA polymerase activity. A "reverse transcriptase" as used herein includes a reverse transcriptase derived from any virus, more particularly from any retrovirus or from any retrotransposon, as well as such reverse transcriptase variants harboring mutations and keeping the reverse transcriptase activities cited above.

As used herein, a "variant" or a "function-conservative variant" includes a nucleic acid sequence in which one or several nucleotides have been changed and which has at least 80 % nucleotide identity as determined by BLAST or FASTA algorithms, preferably at least 90 %, most preferably at least 95%, and even more preferably at least 99 %, and which has the same or substantially similar properties or functions as the native or parent gene to which it is compared.

According to the invention, the reverse transcriptase used in said method is particularly a retroviral reverse transcriptase.

The reverse transcriptase used in the present invention may be for example derived from human immunodeficiency viruses HIV-1 and HIV-2, simian immunodeficiency virus, avian immunodeficiency virus, bovine immunodeficiency virus, feline immunodeficiency virus or equine infectious anemic virus. Preferably, the reverse transcriptase of the invention is a HIV-1 or HIV-2 reverse transcriptase.

According to the invention, the nucleic acid coding for a reverse transcriptase may be manufactured or obtained from a subject sample, said subject being infected by a virus of interest expressing a reverse transcriptase or more particularly a retrovirus of interest.

As used herein, the term "subject" denotes an animal infected by a virus, particularly a retrovirus, more particularly a mammal infected by a retrovirus, such as a rodent, a feline, a canine, and a primate. Preferably, a subject according to the invention is a human. More preferably, the subject is a human patient infected by HIV-1 or HIV- 2.

A nucleic acid sequence coding for a reverse transcriptase obtained from a subject may be obtained by gene amplification (Polymerase Chain Reaction technique, PCR) or by release of the reverse transcriptase by virtue of action of restriction enzymes on purified viral DNA. For this, a biological sample containing the retrovirus genome information is obtained from the subject. For example, if the subject is a human patient infected by the HIV-1 or HIV-2 virus, the nucleic acid sequence coding for HIV-1 or HIV-2 reverse transcriptase may be obtained from the lymphocytes or plasma of a whole blood sample obtained from said patient, or from any other infected organ. Such methods are well known in the art {See for example, the methods disclosed in DWIGHT et al, Journal of Clinical Microbiology 2005, vol43, ni l , pp5696-5704; and in SHAFER et al, Journal of Clinical Microbiology 1996, vol34, n7, ppl 849-1853).

A nucleic acid sequence coding for a reference HIV-1 reverse transcriptase is defined by the nucleic acid sequence SEQ ID NO:l A nucleic acid sequence coding for a reference HIV-2 reverse transcriptase is defined by the nucleic acid sequence SEQ ID NO:2. These both HIV-1 and HIV-2 reference reverse transcriptase are sensitive to NRTI treatment.

On the basis of these nucleic acid sequences, a skilled person can simply design on the basis of its general knowledge primers able to amplify by PCR HIV-1 or HIV-2 reverse transcriptase nucleic acid sequences on DNA obtained from an infected patient sample. As an example, the nucleic acid sequence coding for the HIV-1 or HIV-2 reverse transcriptase may be amplified by PCR from a blood sample of an infected patient using the pair of primers defined by the nucleic acid sequences SEQ ID NO:3 and SEQ ID NO:4 or the pair of primers defined by the nucleic acid sequences SEQ ID NO:5 and SEQ ID NO:6.

In a preferred embodiment, the nucleic acid sequence coding for a reverse transcriptase is under the control of an inducible promoter.

The term "inducible promoter" refers to a transcriptional promoter that promotes transcription of appropriate genes when certain environmental conditions are present. Said promoter is efficient in the yeast cell of the invention. Examples of inducible promoters include, but are not limited to GAL-1 promoter which is inducible by galactose and ADH-2 which is inducible by glucose depletion and MET which is repressed by methionine.

The term "reverse transcription indicator" refers to a marker nucleic acid sequence which permits to check the integration, the expression and the functionality of a nucleic acid sequence coding for a reverse transcriptase inserted into a transformed cell of the invention, preferably in the genome (chromosomal DNA) of the transformed cell. In other terms, a "reverse transcription indicator" according to the invention indicates the presence of a reverse transcriptase activity. In a preferred embodiment, the reverse transcriptase indicator used for the invention is the his3AI gene, which is well known in the art and particularly described in US patents 5,714,313 and 5,462,873.

The US patents 5,714,313 and 5,462,873 describe a method for selecting the transformed yeast cells wherein retrotransposition has occurred, which can be used for the present invention. This method comprises the steps of (i) placing the transformed yeast cells onto a selective medium, (ii) culturing the cells, and (iii) selecting for growing colonies (colonies of the cells which grow). As used herein, the term "deoxycytidine kinase (dCK)" refers to a polypeptide which transfers phosphate to deoxycytidine. dCK is required for the phosphorylation of several deoxyribonucleosides and their nucleoside analogs. The term may include naturally occurring dCK and variants and modified forms thereof. The dCK may be from any species, particularly a mammalian dCK, preferably a human dCK. An exemplary native human dCK mRNA sequence is provided in GenBank database under accession number NM_000788 (SEQ ID NO:7).

As used herein, the term "nucleoside transporter" refers to a large group of membrane transport proteins which transport nucleosides (and particularly nucleosides analogs) across the membranes of cells and/or vesicle. Nucleoside transporters particularly encompass, but are not limited to, equilibrate nucleoside transporters (ENT) and concentrate nucleoside transporters (CNT). According to the invention, the term also encompasses the organic anion transporters (OAT) and the organic cation transporters (OCT).

According to the invention, the nucleoside transporter may for example be selected in the group comprising, but not limited to, CNT1, CNT2, CNT3, ENT1, ENT2, OAT1, OAT3 and OCT1.

In a particular embodiment, the nucleoside transporter is selected in the group comprising, but not limited to, CNT1 , CNT2, CNT3, ENT1, ENT2 and OAT1.

The term ENT1 for equilibrate nucleoside transporter 1 refers to a protein that in humans is encoded by the SLC29A1 gene. The term may include naturally occurring SLC29A1 gene and variants and modified forms thereof. The SLC29A1 gene is typically a mammalian SLC29A1 gene, preferably a human SLC29A1 gene. An exemplary native human SLC29A1 mRNA sequence is provided in GenBank database under accession number NM_001078174 (SEQ ID NO:8).

The term ENT2 {equilibrate nucleoside transporter 2) refers to a protein that in humans is encoded by the SLC29A2 gene. The term may include naturally occurring SLC29A2 gene and variants and modified forms thereof. The SLC29A2 gene is typically a mammalian SLC29A2 gene, preferably a human SLC29A2 gene. An exemplary native human SLC29A2 mRNA sequence is provided in GenBank database under accession number NM_001532 (SEQ ID NO:9).

The term CNT1 (concentrative nucleoside transporter 1) is a protein that in humans is encoded by the SLC28A1 gene. The term may include naturally occurring SLC28A1 gene and variants and modified forms thereof. The SLC28A1 gene is typically a mammalian SLC28A1 gene, preferably a human SLC28A1 gene. An exemplary native human SLC28A1 mRNA sequence is provided in GenBank database under accession number NM_004213 (SEQ ID NO: 10).

The term CNT2 (concentrative nucleoside transporter 2) is a protein that in humans is encoded by the SLC28A2 gene. The term may include naturally occurring SLC28A2 gene and variants and modified forms thereof. The SLC28A2 gene is typically a mammalian SLC28A2 gene, preferably a human SLC28A2 gene. An exemplary native human SLC28A2 mRNA sequence is provided in GenBank database under accession number NM_004212 (SEQ ID NO: 1 1 ).

The term CNT3 (concentrative nucleoside transporter 3) is a protein that in humans is encoded by the SLC28A3 gene. The term may include naturally occurring SLC28A3 gene and variants and modified forms thereof. The SLC28A3 gene is typically a mammalian SLC28A3 gene, preferably a human SLC28A3 gene. An exemplary native human SLC28A3 mRNA sequence is provided in GenBank database under accession number NM 001 199633 (SEQ ID NO:12).

The term OAT1 (organic anion transporter 1) is a protein that in humans is encoded by SLC22A6 gene. The term may include naturally occurring SLC22A6 gene and variants and modified forms thereof. The SLC22A6 gene is typically a mammalian SLC22A6 gene, preferably a human SLC22A6 gene. An exemplary native human SLC22A6 mRNA sequence is provided in GenBank database under accession number NM_004790 (SEQ ID NO: 13).

The term OAT3 (organic anion transporter 3) is a protein that in humans is encoded by SLC22A8 gene. The term may include naturally occurring SLC22A8 gene and variants and modified forms thereof. The SLC22A8 gene is typically a mammalian SLC22A8 gene, preferably a human SLC22A8 gene. An exemplary native human SLC22A8 mRNA sequence is provided in GenBank database under accession number NM_001 184732 (SEQ ID NO:14).

The term OCT1 (organic cation transporter I) is a protein that in humans is encoded by SLC22A1 gene. The term may include naturally occurring SLC22A1 gene and variants and modified forms thereof. The SLC22A1 gene is typically a mammalian SLC22A8 gene, preferably a human SLC22A1 gene. An exemplary native human SLC22A1 mRNA sequence is provided in GenBank database under accession number NM_003057 (SEQ ID NO: 15).

According to the invention, the transformed yeast cell may comprise in its genome one, two, three, four, five, six, seven or eight nucleic acids sequences coding for nucleoside transporters.

In one embodiment of the invention, the transformed yeast cell of the invention at least comprises the nucleic acid sequence coding for OAT1.

In fact, the inventors have established that the expression of OAT1 enables to obtain a good inhibition of HIV RT by Zidovudine, Stavudine, Didanosine or Zalcitabine. In a particular embodiment, said transformed yeast cell at least comprises one more nucleic acid sequence coding for another nucleoside transporter.

In another embodiment of the invention, the transformed yeast cell of the invention at least comprises the nucleic acid sequence coding for CNT3.

In fact, the inventors have established that the expression of CNT3 enables to obtain a good inhibition of HIV RT by AZT (Zidovudine) or by d4T (Stavudine). In a particular embodiment, said transformed yeast cell at least comprises one more nucleic acid sequence coding for another nucleoside transporter. In a particular embodiment of the invention, the transformed yeast cell of the invention further comprises a nucleic acid sequence coding for a thymidine kinase.

As used herein, the term "thymidine kinase" (TK) has its general meaning in the art and refers to a kinase (which is required for the action of many antiviral drugs). TK is required for the phosphorylation of several nucleoside analogs. The term may include naturally occurring TK and variants and modified forms thereof. The TK may be from any species. The TK may be for example a human TK, but also a viral TK. Typically, the TK used according to the invention may be the TK from Human Herpes Simplex Virus Type 1 HSV1-TK (SEQ ID NO: 16).

Methods of the invention

The transformed yeast cells of the invention may be used for improving and ging therapies using Nucleoside Reverse Transcriptase Inhibitors (NRTI). Thus, a second object of the invention relates to a method for screening a compound with the ability to inhibit reverse transcription comprising the steps of:

i. contacting a transformed yeast cell containing a reverse transcription indicator according to the invention with a compound,

ii. culturing said transformed yeast cell in a selective medium, iii. detecting inhibition of growth of the transformed yeast cell compared to said transformed yeast cell that is not contacted with said compound, and

iv. selecting a compound with the ability to inhibit growth of the transformed yeast cell as being a compound that inhibit reverse transcription.

According to the invention, the term "inhibition of growth" includes a significant decrease in growth compared to cells that are not contacted with the screened or tested compound. It thus includes any relative inhibition of growth that can be quantified.

In a particular embodiment, said inhibition of growth refers to a decrease of at least 30%, more particularly of at least 50%, even more particularly of at least 60%, preferably of at least 70%, more preferably of at least 80%, even more preferably of at least 90% in growth compared to cells that are not contacted with the screened or tested compound, wherein said compound is used at the higher concentration permitting its solubility in aqueous medium.

According to the invention, the reverse transcription indicator permits to the transformed yeast cell of the invention to grow in particular conditions in which non- transformed yeast cells (or transformed yeast cell in which reverse transcription does not occur) could not grow. The reverse transcription indicator thus enables selection of transformed yeast cells in which reverse transcription occurs, which are of interest for the invention.

The selective medium is chosen so as to select yeast cells able to grow in said particular conditions, in which reverse transcription effectively occurs. For example, in the case of use of his3 AI as reverse transcription indicator, yeast cells in which the reverse transcription occurs are able to grow in a medium lacking histidine. Thus, selective medium used with the his3AI gene as reverse transcription indicator is a medium lacking in histidine.

According to the invention, a transformed yeast cell of the invention which has been contacted with a tested NRTI compound grows in a classical cell culture medium (which is not selective).

For studying the effect of the tested NRTI compound on reverse transcription, said compound has to be added before the reverse transposition occurred.

So, preferably, the nucleic acid sequence coding for reverse transcriptase is under the control of an inducible promoter and reverse transcription (e.g. inducing the expression of the nucleic acid sequence encoding reverse transcriptase) is induced after contacting the tested NRTI compound with the transformed yeast cell of the invention.

Thus, in a preferred embodiment, the invention relates to a method for screening a compound with the ability to inhibit reverse transcription comprising the steps of:

i. contacting a transformed yeast cell containing a reverse transcription indicator and a reverse transcriptase under control of an inducible promoter according to the invention with a compound,

ii. inducing reverse transcription,

iii. culturing said transformed yeast cell in a selective medium, iv. detecting inhibition of growth of the transformed yeast cell compared to said transformed yeast cell that is not contacted with said compound, and

v. selecting a compound with the ability to inhibit growth of the transformed yeast cell as being a compound that inhibit reverse transcription. According to the method of the invention, compounds that could be tested according to the invention have a chemical structure similar to nucleosides and known NRTI. According to the invention, the compounds selected according to this method can be used for developing therapeutic strategies against the retrovirus from which the reverse transcriptase of the transformed yeast cell is derived.

According to the invention, said reverse transcriptase may be from retrotransposons or viruses; particularly retroviruses.

Reverse transcriptases encoded in retrotransposon elements are implicated in cancer mechanisms and viral reverse transcriptases (more particularly retroviral reverse transcriptases) in viral infections.

In a particular embodiment, said reverse transcriptase may be from retroviruses. In this case, the method of the invention permits to detect new compounds inhibiting retroviral replication.

According to the invention "inhibiting retroviral replication" can be used interchangeably with "inhibiting the reverse transcriptase" or "inhibiting reverse transcription" in the case of a retroviral reverse transcriptase.

In a more particular embodiment of the invention, the reverse transcriptase of the transformed cell used in this method is derived from a retrovirus selected in the group comprising HIV-1 and HIV -2, simian immunodeficiency virus, avian immunodeficiency virus, bovine immunodeficiency virus, feline immunodeficiency virus or equine infectious anemic virus. Preferably, the reverse transcriptase is derived from HIV-1 or HIV-2.

In another preferred embodiment, compounds selected by the methods of the invention may further be tested in a model of infection by the retrovirus from which the reverse transcriptase of the transformed yeast cell is derived. A third object of the invention relates to a method for predicting the sensitivity of a reverse transcriptase to a NRTI compound, said method comprising the steps of:

i. producing a transformed yeast cell as previously described, comprising a reverse transcriptase indicator and a reverse transcriptase to be studied,

ii. contacting said transformed yeast cell with said NRTI compound,

iii. culturing the transformed yeast cell in a selective medium, iv. determining the growth of the transformed yeast cell with or without NRTl compound, and

v. deducing therefrom if the reverse transcriptase to be studied is sensitive or resistant to said NRTl compound.

In one embodiment, an inhibition of growth after contacting the transformed yeast cells with the NRTl compound indicates that the reserve transcriptase to be studied is sensitive to said NRTl compound.

According to the invention, the term "inhibition of growth" includes any decrease in growth in said transformed yeast cell comprising a reverse transcriptase to be studied (tested transformed yeast cell) compared to a transformed yeast cell of the invention containing a reference reverse transcriptase known to be sensitive (reference transformed yeast cell).

Such an inhibition may be determined by measuring the ratio of the IC ₅₀ values of the contacted NRTl compound for the tested transformed yeast cell versus for a reference transformed yeast cell (IC ₅₀ (studied RT/ IC ₅₀ (reference RT))-

According to the invention, a ratio ICso (studied RT IC ₅₀ (reference RT) lower than or equal to 1 indicates that the reverse transcriptase to be studied is sensitive to said NRTl compound; a ratio IC ₅₀ (studied RT/ IC ₅₀ (reference RT) higher than 1 indicates that the reverse transcriptase to be studied is less sensitive to said NRTl compound than the reference reverse transcriptase.

Thus, in a preferred embodiment, said method comprises a further step after step (iv) consisting in determining the ratio of the IC ₅₀ values of the contacted NRTl compound for the tested transformed yeast cell versus the IC ₅₀ values of the contacted NRTl compound for a reference transformed yeast cell containing a reference reverse transcriptase known to be sensitive to said NRTl compound.

According to said preferred embodiment, a ratio lower than or equal to 1 indicates that the reserve transcriptase to be studied is sensitive to said NRTl compound. According to the invention, the reverse transcriptase to be studied may be derived from a retrotransposon or a virus, particularly a retrovirus. Particularly, said reverse transcriptase is derived from a retrovirus.

For example, the reverse transcriptase to be studied in this method may be derived from a retrovirus selected in the group comprising HIV-1 and HIV-2, simian immunodeficiency virus, avian immunodeficiency virus, bovine immunodeficiency virus, feline immunodeficiency virus or equine infectious anemic virus.

Thus, in a particular embodiment, the invention relates to a method for predicting the sensitivity of a reverse transcriptase derived from a retrovirus infecting a subject to a NRTI compound, said method comprising the steps of:

i. producing a transformed yeast cell as previously described, wherein the reverse transcriptase is derived from a retrovirus infecting said subject,

ii. contacting said transformed yeast cell with said NRTI compound,

iii. culturing the transformed yeast cell in a selective medium, iv. determining the growth of the transformed yeast cell with or without NRTI compound, and

v. deducing therefrom if the reverse transcriptase derived from the retrovirus infecting said subject is sensitive or resistant to said NRTI treatment.

Preferably, said reverse transcriptase is a HIV-1 or HIV-2 reverse transcriptase (and said subject is a human patient infected by HIV-1 or HIV-2).

In one embodiment, an inhibition of growth after contacting the transformed yeast cells with the NRTI compound indicates that the reserve transcriptase of the retrovirus infecting the subject, and the retrovirus itself, is sensitive to said NRTI compound. Thus, the subject is likely to respond to said NRTI compound.

According to the invention, the term "inhibition of growth" includes any decrease in growth in the transformed yeast cell comprising the reverse transcriptase derived from a retrovirus infecting said subject (tested transformed yeast cell) compared to a transformed yeast cell of the invention containing a reference reverse transcriptase of said retrovirus known to be sensitive (reference transformed yeast cell). Such an inhibition may be determined by determining the ratio of the IC ₅ o values of the contacted NRTI compound for the tested transformed yeast cell versus the IC ₅₀ values of the contacted NRTI compound for a reference transformed yeast cell (IC ₅₀

(subject RT) IC ₅₀ (reference RT))- According to the invention, a ratio IC ₅₀ (subject RT/ IC50 (reference RT) lower than or equal to 1 indicates that the reverse transcriptase to be studied is sensitive to said NRTI compound; the subject is likely to respond to a treatment based on said NRTI compound. A ratio IC50 (studied RT/ IC ₅₀ (reference RT) higher than 1 indicates that the reverse transcriptase to be studied is less sensitive to said NRTI compound than the reference reverse transcriptase; the subject is at risk to not respond to a treatment based on said NRTI compound.

According to the invention, HIV-1 and HIV-2 reference reverse transcriptase may be respectively defined by nucleic acid sequences SEQ ID NO: l and SEQ ID NO:2.

Preferably, the nucleic acid sequence coding for reverse transcriptase is under the control of an inducible promoter and reverse transcription (e.g. inducing the expression of the nucleic acid sequence encoding reverse transcriptase) is induced after contacting the tested NRTI compound with the transformed yeast cell of the invention.

According to the invention, a transformed yeast cell of the invention which has been contacted with a tested NRTI compound grows in a classical cell culture medium (which is not selective).

In still another embodiment of the invention, the reverse transcription indicator used in the transformed yeast cell is his3AI, and the selective medium used on step (ii) is a cell medium lacking in histidine.

In a preferred embodiment, said NRTI compound may be selected in the group comprising or consisting of Zidovudine, Didanosine, Stavudine, Lamivudine, Abacavir, Emtricitabine, Tenofovir and Zalcitabine. In a particular embodiment, combinations of NRTI compounds may be tested. For example, combination of Tenofovir, Lamivudine, Abacavir and Emtricitabine, combination of Stavudine, Zidovudine and Didanosine could be tested. In one embodiment, the invention relates to said method for predicting the resistance or sensitivity of a reverse transcriptase of a reverse transcriptase to a NRTI compound, wherein the NRTI to be tested is Zidovudine or Stavudine and wherein the transformed yeast cell comprises at least a nucleic acid coding for the nucleoside transporter CNT3.

In another particular embodiment, the invention relates to said method for predicting the resistance or sensitivity of a reverse transcriptase of a reverse transcriptase to a NRTI compound, wherein the NRTI to be tested is Zidovudine, Stavudine, Didanosine or Zalcitabine and wherein the transformed yeast cell comprises at least a nucleic acid sequence coding for the nucleoside transporter OAT1.

EXAMPLES

The following examples describe some of the preferred modes of making and practicing the present invention. However, it should be understood that the examples are for illustrative purposes only and are not meant to limit the scope of the invention.

Example 1: Activity of thymidine analog reverse-transcriptase inhibitors on RT in strain carrying dCK and CNT3.

S. cerevisiae strain A (Mat a, URA-, HIS-, LEU-) was transformed, using the standard Lithium Acetate protocol, with a PCR amplified DNA fragment having the nucleic acid sequence coding for the ura3 yeast gene. The transformed cell, Strain Al (Mat a, URA+, HIS-, LEU-) has integrated the ura3 gene in its genomic locus. Strain Al (Mat a, URA+, HIS-, LEU-) was transformed, using the standard Lithium Acetate protocol, with a previously linearized by Stul restriction enzyme p426GPDhdCK plasmid that contains the nucleic acid sequence coding for human dCK and ura3, and where linearization occurred within the ura3 open reading frame. The new strain (Strain A#), where integration of dCK occurred at the ura3 locus has the following phenotype (Mat a, URA-, HIS-, LEU-, dCK+). Strain A# was transformed with an expression vector plasmid containing the nucleic acid sequence coding for the nucleoside transporter CNT3, and the plasmid pHART21 described in US patent 5,714,313. HIV-1 RT activity in the resulted yeast strain and in non-modified Strain A was determined as described in US patent 5,714,313 after 72 hours incubation in the presence or the absence of 200μΜ of either AZT (Zidovudine) or d4T (Stavudine). NRTI dependent inhibition of HIV-1 RT took place when the foreign proteins were present (Table 1).

Table 1. Expression of dCK and CNT3 allows inhibiting HIV Reverse Transcriptase activity in yeast cells.

Example 2: RT activity and inhibitor susceptibility in strain carrying dCK in its genome and OAT1 in a plasmid.

Strain A# (Mat a, URA-, HIS-, LEU-, dCK+) was transformed with an expression vector plasmid containing the nucleic acid sequence coding for the nucleoside transporter OAT1 , and the plasmid pHART21 described in US patent 5,714,313. HIV-1 RT activity in the resulted yeast strain and in non-modified Strain A (Mat a, URA-, HIS-, LEU-) was determined as described in US patent 5,714,313 after 72 hours incubation in the presence or the absence of ΙΟΟμΜ of either AZT, ddC (Zalcitabine), ddl (Didanosine) or d4T. NRTI dependent inhibition of HIV-1 RT took place when the foreign proteins were present (Figure 1 ).

Example 3: RT activity and inhibitor susceptibility in strain carrying dCK and OAT1 in its genome.

S. cerevisiae strain B (Mat a, URA+, HIS-, LEU+) was transformed, using the standard Lithium Acetate protocol, with a previously linearized by Stul restriction enzyme p426GPDhdCK plasmid that contains the nucleic acid sequences coding for dCK and ura3, and where linearization occurred within the ura3 open reading frame. The new strain (Strain B#) where integration of dCK occurred at the ura3 locus has the following phenotype (Mat a, URA-, HIS-, LEU+, dCK+). Strain B# was genetically modified by standard methods through transformation with a PCR amplified DNA fragment. This nucleic acid fragment contains the nucleic acid sequence coding for OATl transporter flanked by 5' and 3' non-coding regions of the leu2 locus. The new modified strain where dC and OATl are integrated in the yeast genome, Strain B## Mat a, URA-, HIS-, LEU-, dCK+, OAT1+), was transformed with the plasmid pHART21 described in US patent 5,714,313. HIV-1 RT activity in Strain B## was determined as described in US patent 5,714,313 after 72 hours incubation in the presence of ΙΟΟμΜ of either AZT, ddC, ddl or d4T. NRTI dependent inhibition of HIV-1 RT took place when the foreign proteins were present (Figure 2), showing that activity of those proteins is not related to the way they are expressed (as plasmid, Figure 1 , or in the yeast genome, Figure 2).

Example 4: Determining IC50 of NRTIs in yeast strain carrying dCK and OATl in its genome (Strain B##).

Determination of IC50 values of NRTIs to HIV-1 RT have been performed in

Strain B## according to the method described in US patent 5,714,313. After 72 hours incubation in the presence or the absence of different concentrations of either ddl, AZT, TFV (Tenofovir), ddC or d4T, cell growth was measured at OD at 600nm. IC ₅₀ values, defined as the amount (concentration) of NRTI inhibiting yeast growth to half (50%), were determined from plotting the percentage of growth inhibition versus NRTI concentration, expressed in logarithm scale (Figure 3).

Example 5: Non-efficiency of a strain carrying NT5C and nucleoside transporters compared to strain carrying dCK and nucleoside transporters. The cytosolic 5 'nucleotidase, NT5C, has been shown to have a phosphotransfer capacity (Johnson MA, Fridland 1989 A.Mol Pharmacol. 36:291-5 Phosphorylation of 2',3'-dideoxyinosine by cytosolic 5 '-nucleotidase of human lymphoid cells) involved in NRTI phosphorylation. This protein was thus tested in a transformed yeast cell of the invention as a "dCK-like protein" and a construction similar to the invention comprising RT, RT indicator, NT5C and a nucleoside transporter was performed. The nucleic acid sequence coding for the human cytosolic 5 'nucleotidase, NT5C was integrated in the yeast genome as follows. S. cerevisiae strain Al (Mat a, URA+, HIS-, LEU-) was transformed, using the standard Lithium Acetate method, with a PCR amplified DNA fragment comprising a nucleic acid sequence identical to a 50 nucleotides length sequence upstream the start codon of the yeast ura3 gene, followed by the GPD promoter sequence, followed by the nucleic acid sequence coding for the 5' nucleotidase, followed by a nucleic acid sequence identical to a 50 nucleotides length sequence downstream the stop codon of the yeast ura3 gene. The modified yeast strain, Strain A§ (Mat a, HIS-, LEU-, URA-, NT5C+) where the integration of the foreign gene occurred at the ura3 locus, was further transformed, following standard methods, with a plasmid harboring the nucleic acid coding sequence of the human nucleotide transporter CNT2 under the control of the ADH promoter. This strain was named Strain A§§( Mat a, HIS-, LEU+, URA-, NT5C+, CNT2+).

S. cerevisiae strain A# (Mat a, URA-, HIS-, LEU-, dCK+) from example 1, was transformed, using standard methods, with a plasmid harboring the nucleic acid coding sequence of the human nucleotide transporter CNT2 under the control of the ADH promoter. This strain was named Strain A#§ (Mat a, URA-, HIS-, LEU+, dCK+, CNT2+).

HIV RT activity was determined in both strains as described in US patent 5,714,313 as follows. Both HIV1 RT expressing strains were grown in SD-ura+glucose to saturation. 1DO of saturated culture (about 10 ⁷ cells) was seeded in 1ml SGal-ura and grew for 6 hrs. Glucose was then added to stop RT activation. After 12 hours, induced cells were seeded at 0.2DO/ml in SD-His media and incubated for 72 hours. Incubations were performed at 30°C with shaking. Inhibition of HIV-1 RT activity was determined by measuring the inhibition of growth in the presence 200μΜ concentration of ABC (Abacavir) or TFV (Tenofovir) inhibitors, as shown in Table II. The obtained results proved that the presence of dCK but not the presence of NT5C in yeast led to a substantial activity of NRTI inhibitors.

% Inhibition by ABC % Inhibition by TFV

Strain A§§ (NT5C+, CNT2+) 0% 0%

Strain A#§ (dC +, CNT2+) 87% 96% Table 2: HIV-1 Reverse transcriptase inhibition by Abacavir (ABC) and Tenofovir (TFV) determined in modified yeast cells.