In Vitro Selection and Characterization of Human Immunodeficiency Virus Type 1 Variants with Increased Resistance to ABT-378, a Novel Protease Inhibitor

The human immunodeficiency virus (HIV) contains an aspartyl protease whose function is required for the proper processing of Gag and Gag-Pol polypeptide precursors into the structural proteins of the virus (MA [p17], CA [p24], NC [p7], and p6) as well as the enzymes necessary for viral propagation (reverse transcriptase [RT], integrase [IN], and protease) (29). Because inhibition of the HIV protease is known to result in the formation of noninfectious viral particles (18, 21), the HIV protease has long been considered a good therapeutic target for the treatment of patients with AIDS (10). Much progress has been made in recent years in the development of compounds that specifically inhibit this enzyme, and there are currently four protease inhibitors licensed for the treatment of patients infected with HIV. These compounds have greatly enhanced the repertoire of drugs available to HIV patients and have helped foster the hope that infection with HIV may someday be a successfully treated condition. In spite of this remarkable progress, one of the most serious hurdles facing the successful clinical use of these compounds is the suppression of drug-resistant variants of HIV. Resistance to protease inhibitors has been observed in vitro and is due to specific mutations in the protease that lead to decreased drug sensitivity (11, 13, 16, 17, 20, 22, 23, 25, 28, 36). Not unexpectedly, similar mutations have also been observed in vivo, leading to viral resistance in patients receiving therapy with these compounds (6, 7, 15, 24, 40). Because of these limitations, it is important to investigate novel protease inhibitors that are not only more potent, but whose resistance profiles differ from those of the currently available compounds. ABT-378 is a novel protease inhibitor, structurally related to ritonavir (ABT-538) (19, 22, 24), that is currently in clinical development (Fig. ​(Fig.1).1). This compound is significantly more active than ritonavir in cell culture, even in the presence of human serum proteins (35). Although ABT-378 produces a plasma drug profile that is similar to that of most other protease inhibitors when dosed alone, when codosed with small amounts of ritonavir, this compound achieves and maintains plasma drug levels that are highly suppressive of HIV replication in vitro (35). Additionally, this compound retains high antiviral activity against ritonavir-resistant strains of HIV (35). In this study, we describe the in vitro selection and characterization of HIV-1 variants having increased resistance to ABT-378. Specific mutations in the protease as well as in two of the gag proteolytic cleavage sites were characterized and were shown to be important in conferring resistance to this compound. The results observed during in vitro selection with ABT-378 may be predictive of possible resistance patterns observed in vivo and may aid in the clinical development and therapeutic utility of this compound. FIG. 1 Structure of ABT-378. Ph, phenyl.