Exploiting Drug Repositioning for Discovery of a Novel HIV Combination Therapy

The development of HIV drugs is an expensive and a lengthy process. In this study, we used drug repositioning, a process whereby a drug approved to treat one condition is used to treat a different condition, to identify clinically approved drugs that have anti-HIV activity. The data presented here show that a combination of two clinically approved drugs, decitabine and gemcitabine, reduced HIV infectivity by 73% at concentrations that had minimal antiviral activity when used individually. Decreased infectivity coincided with a significant increase in mutation frequency and a shift in the HIV mutation spectrum. These results indicate that an increased mutational load is the primary antiviral mechanism for inhibiting the generation of infectious progeny virus from provirus. Similar results were seen when decitabine was used in combination with another ribonucleotide reductase inhibitor. Our results suggest that HIV infectivity can be decreased by combining a nucleoside analog that forms noncanonical base pairs with certain ribonucleotide reductase inhibitors. Such drug combinations are relevant since members of these drug classes are used clinically. Our observations support a model in which increased mutation frequency decreases infectivity through lethal mutagenesis.There are more than 20 drugs approved for the treatment of HIV infection. However, the efficacy of these drugs is limited by drug resistance, which emerges when drug levels are not high enough to sufficiently inhibit viral replication. While there are currently five classes of HIV therapy, a mutation that confers resistance to one drug often confers resistance to other members of the same drug class. Thus, the emergence of drug resistance limits potential drug therapies, making new anti-HIV therapies essential for successful long-term treatment of HIV infection. However, the development of novel anti-HIV drugs is costly (∼$600 million) and time-consuming (over 12 years) (12). One way to decrease the cost and expedite the development of novel drugs is to use a drug repositioning strategy which involves using drugs that are clinically approved for one condition to treat a different condition (1). Drug repositioning expedites drug development by making use of drugs whose toxicity and pharmacokinetic profiles have already been thoroughly characterized. Such a strategy has been successfully used for the treatment of conditions such as cancer, obesity, and osteoporosis, as well as others (1). For example, zidovudine (AZT), which is clinically approved for the treatment of HIV infection, was originally developed as an anticancer drug (20, 24). Thus, to expedite the development of novel anti-HIV drugs, we examined clinically approved drugs for the ability to inhibit HIV infectivity.We focused on clinically approved antimetabolites (Table ​(Table1)1) for two reasons. First, none of the current anti-HIV drugs are antimetabolites. Therefore, any compounds identified as having anti-HIV activity would likely offer a new mechanism of action. Second, antimetabolites have been shown to have activity against a wide variety of viruses, such as poliovirus and foot-and-mouth disease virus (FMDV) (34, 35, 37). This antiviral activity is likely attributable to either a reduction in viral replication or an increase in the viral mutation rate. The ability of antimetabolites to reduce replication is likely due to a reduction in deoxynucleoside triphosphate (dNTP) pools, which are required for viral replication (2, 3, 10, 28). Alternatively, alterations of dNTP pools by antimetabolites have been shown to increase the HIV mutation rate, which correlates with a loss of infectivity. This loss of infectivity has been attributed to the process of lethal mutagenesis, a term used to describe the idea that the mutation rate can surpass a threshold beyond which the virus is unable to replicate its genome with enough fidelity to remain infectious. Although an inverse correlation between mutation frequency and infectivity has been shown for a number of viruses, there are few, if any, drugs used clinically that specifically target viral mutation rates.

TABLE 1.

Compounds screened for anti-HIV activity^a

Open in a separate window

Open in a separate window^aAll compounds were screened for anti-HIV activity alone or in combination with decitabine using the single-cycle HIV assay shown in Fig. ​Fig.11.In this study, we describe the identification of a novel combination therapy for HIV infection composed of two nucleoside analogs that are clinically approved for the treatment of precancerous or cancerous states. The two drugs, decitabine and gemcitabine, significantly decrease HIV infectivity when used individually. However, when used in combination, the drugs worked synergistically to decrease or eliminate HIV infectivity without any detectable effect on cell proliferation. Similar results were seen when decitabine was used in combination with another ribonucleotide reductase inhibitor, hydroxyurea. We provide data that suggest that the combination therapy targets the mutation rate of HIV, a drug target that has yet to be exploited clinically. Importantly, these results reveal a novel therapeutic strategy to inhibit HIV replication. Specifically, we show here that HIV infectivity can be synergistically decreased by combining two classes of compounds, (i) nucleoside analogs that form noncanonical base pairs and (ii) certain ribonucleotide reductase inhibitors. Furthermore, since many of the drugs from each of these drug classes are already clinically approved, it is likely that such a drug combination is clinically relevant to the treatment of HIV infection.