In Vitro Antiviral Activity and Cross-Resistance Profile of PL-100, a Novel Protease Inhibitor of Human Immunodeficiency Virus Type 1

At the end of 2005, an estimated 38.6 million people worldwide were living with human immunodeficiency virus (HIV), with approximately 4.1 million cases of new infections and 2.8 million deaths due to AIDS (32). Highly active antiretroviral therapy (HAART) has resulted in durable virological suppression and a marked decrease in morbidity and mortality associated with HIV, bearing testimony to the success of HAART (17, 31, 33) in Western countries, in which access to therapeutic drugs is guaranteed. However, the development of viral resistance is a major cause of treatment failure (2, 12, 21, 31, 34). Mutated, drug-resistant HIV type 1 (HIV-1) strains emerge through the combined effects of the lack of proofreading activity of the viral reverse transcriptase (RT), recombination between coinfecting isolates (3, 27), and the high replication rate of HIV in vivo (9, 35). Drug-resistant HIV is a major clinical problem, not only for patients for whom therapy fails, but for drug-naive patients, as well. In North America and Europe, it is estimated that approximately 10% of new HIV infections harbor drug-resistant mutations (15, 25, 37). Thus, novel therapeutic drugs with activity against resistant strains are needed. Other barriers to effective treatment are the toxicity of the drugs taken daily for the rest of a patient's life and the correlated lack of adherence to treatment. Therefore, new compounds should be highly specific, potent, and sufficiently bioavailable to limit the pill burden, in addition to being nontoxic. HIV-1 protease (PR) has been recognized as a therapeutic target since the approval of the first PR inhibitor (PI) in 1995. Inhibition of this 99-amino-acid homodimeric enzyme prevents the proteolytic processing of the Gag and Gag-Pol viral polyproteins into the structural proteins (p17, p24, p2, p7, p1, and p6) and the viral enzymes (PR, RT, and integrase), thereby blocking viral infectivity (14). Hence, PIs have become cornerstones in the treatment of AIDS as components of HAART both for first-line medications in treatment-naive patients and in patients with a long history of antiretroviral therapy. However, HIV can develop resistance to specific PIs through selection of amino acid substitutions in PR itself. Many mutated residues have been shown to decrease the enzyme's binding affinity for the inhibitors while the ability of PR to cleave its substrates is preserved. Distinct key or signature mutations have been associated with resistance to specific PIs (12, 18). In addition to these so-called primary mutations, other mutations, generally further away from the catalytic site, also play significant roles in resistance. However, the exact roles of these so-called compensatory or secondary mutations is not always clearly defined, although a role in enzymatic and viral fitness has been demonstrated for some of them. Moreover, some mutations in PR confer cross-resistance among multiple PIs. Often, drug selective pressure may drive the accumulation of several primary mutations against a background of particular secondary mutations to favor the emergence of cross-resistance (12). This mainly involves amino acid substitutions in PR at positions 10, 32, 46, 54, 82, 84, and 90 (8, 12). Thus, a priority in antiretroviral-drug research is now the development of new HIV inhibitors that exhibit distinct resistance profiles to provide patients with alternatives in combination therapy. To tackle this challenge, a drug discovery program was established that integrated viral resistance directly into the screening process (26, 28-30). We present the biochemical and virological characterization of a new PI, termed PL-100, that emerged from this program. PL-100 is a novel, specific, and noncytotoxic inhibitor of the HIV-1 PR that shows good antiviral activity against both wild-type laboratory strains and a wide spectrum of PI-resistant isolates.