Cell-based and biochemical screening approaches for the discovery of novel HIV-1 inhibitors

Abstract

The identification of novel HIV-1 inhibitors is facilitated by screening campaigns that combine the right screening strategy with a large diverse collection of drug-like compounds. Cell-based screening approaches offer some advantages in the quest for novel inhibitors because they can include multiple targets in a single screen and in some cases reveal targets and/or structures not captured in biochemical assays. However, follow-up activities for cell-based screens are often more complicated and resource intensive when compared to biochemical screens. Alternatively, biochemical screens usually offer the advantage of focusing on a single target with a well-defined set of follow-up assays. In this review we cover multiple cell-based and biochemical assay formats, many of which were designed to identify inhibitors that act through new mechanisms. Some of the assays discussed have been utilized in antiviral screens while others might be formatted for HTS or utilized as secondary assays in a screening campaign. As drug discovery efforts in the pharmaceutical industry shift away from traditional strategies, new approaches such as those presented here are likely to play a significant role in the identification of next generation HIV-1 inhibitors.

Introduction

Presently, there are 20 individual drugs that have been approved to treat HIV-1 infection. However, each of those 20 drugs target one of only three steps in the HIV-1 replication cycle (HIV-1 fusion, reverse transcriptase, or protease). Given that viral variants resistant to one drug of a particular class often exhibit some level of cross-resistance to other drugs within the same class, therapeutic options are often limited in treatment experienced patients. To address this problem, HIV researchers in the pharmaceutical industry have concentrated their efforts in recent years on discovery programs designed to identify antiviral agents effective against both wild-type and drug-resistant HIV-1 variants. These efforts have resulted in the progression of compounds that either act via an established mechanism but have a novel resistance profile (next generation inhibitors) or that act through new mechanisms. Recently, proof-of-concept regarding clinical efficacy has been demonstrated for three new targets in the HIV-1 replication cycle (HIV-1 coreceptors, HIV-1 gp120 and HIV-1 integrase) (Hanna et al., 2004, Fätkenheuer et al., 2004, Hendrix, 2004, Little et al., 2005, Reynes et al., 2002) and suggested for a fourth (virion maturation) (Martin et al., 2005). Whilst this is encouraging, the long-term clinical safety and efficacy of these agents remains to be determined. In addition, drug resistance will most likely remain a recurrent problem in chronic antiviral therapy. Therefore, the continued discovery and development of new HIV-1 inhibitors that will be effective in future antiretroviral regimens is critical.

Multiple screening approaches are currently available for HIV-1 drug discovery, and several different approaches have been used successfully to identify new HIV-1 inhibitors. For example, nucleoside analog reverse transcriptase inhibitors (NRTIs), the first non-nucleoside reverse transcriptase inhibitors (NNRTIs), and several recent novel target inhibitors were discovered using virus-based screening approaches. Alternatively, protease inhibitors (PIs), next generation NNRTIs, CCR5 antagonists, and integrase inhibitors were identified by structure-based drug design, receptor pharmacology or biochemical screening approaches. Therefore, historical precedent suggests that diverse screening strategies should be employed for the discovery of new HIV-1 agents. In the sections below, we present a brief overview of various HIV-1 screening strategies and highlight novel approaches and/or significant advances in HIV-1 screening technology.