Drug resistance to human immunodeficiency virus (HIV) is a major factor in the failure of antiretroviral therapy.1 In order for practitioners to provide effective pharmaceutical care to their HIV patients, it is essential that they understand the mechanisms of HIV drug resistance as well as the various factors that can contribute to its emergence. This article is based on didactic content from the infectious disease section of the Integrated Sequence II Course in the PharmD program at South University. In the course, students are first given an overview that includes key structural components of HIV and a discussion of the HIV life cycle. A detailed presentation on the pharmacology of the various classes of antiretroviral agents follows. The clinical impact and prevalence of HIV drug resistance is then discussed along with factors that might contribute to it. Mechanisms of drug resistance for each class of antiretroviral agents are presented in detail followed by a discussion of the basis and clinical utility of HIV drug resistance testing. Finally, new targets for HIV pharmacotherapy are presented along with an overview of new antiretroviral agents that are being developed. Content taught in lecture is reinforced by relevant case studies that students work on in small groups during the recitation period.
Keywords: human immunodeficiency virus, HIV pharmacotherapy, drug resistance, pharmacology
INTRODUCTION
Since its introduction, antiretroviral drug therapy has dramatically reduced morbidity and mortality associated with HIV infection. The use of combination drug therapies can significantly improve HIV patients' chances for longterm survival. Unfortunately, the effectiveness of antiretroviral therapy can bemarkedly reduced by theemergence of drug resistance. A study by Richman2 in the journal AIDS reported that 76% of their population exhibited resistance to 1 ormore antiretroviral drugs.While exposure to antiretroviral drugs can contribute to the development of resistance by HIV, even drug-naïve patients may be infected with strains of the virus that are resistant to drug therapy.3 The presence of antiretroviral drug resistance is an important cause of treatment failure in HIV patients. Drug resistant viruses are often resistant tomultiple classes of antiretroviral drugs. This drug cross-resistance coupled with the often unpredictable development of drug resistance significantly complicates HIV therapy. Successful treatment of HIV requires a detailed knowledge of the various mechanisms by which resistance can arise as well as an understanding of strategies for overcoming resistance once it occurs. HIV drug resistance testing is proving to be a powerful tool that can help clinicians tailor their treatment regimen to the specific HIV strain(s) that infect their patients. In addition, numerous new agents and classes of antiretroviral drug are currently under development in an effort to keep pace with emerging HIV drug resistance.
INSTRUCTIONAL DESIGN
At South University the presentation on HIV drug resistance is given during our Integrated Sequence II (ISII) course which is an 8 quarter-hour block of integrated instruction in pharmacology, medicinal chemistry, and therapeutics. At this point in the program, our students have completed a course in Medical Microbiology and are familiar with the fundamental areas of pharmacology and medicinal chemistry taught in Integrated Sequence I. Within the ISII course itself, the material on HIV drug resistance is presented after students have received instruction on pharmacology andmedicinal chemistry related to antiretroviral drugs therapy and before the clinical faculty member begins his/her presentation of antiretroviral therapeutics. Since students have just learned the mechanism of action for each of the antiretroviral drug classes it is a logical extrapolation to the mechanism of resistance for each class. When the clinical faculty member presents antiretroviral therapeutics, they can do so knowing that students have an understanding of drug resistance mechanisms, factors that contribute to the development of resistance, and the importance of HIV drug-resistance testing. Student knowledge related to antiretroviral drug pharmacology and resistance is reinforced by the presentation of relevant case studies during a weekly 3-hour recitation period. Students are given cases at the beginning of the period and then broken into small pre-assigned groups to work through the cases. At the end of each case is a series of questions addressing various aspects of therapeutics, pharmacology, medicinal chemistry, and pathophysiology related to HIV infection and treatment. At the end of 2 hours, each group is expected to write the case and submit it for review and grading. Each member of the group receives the same grade for the case write-up and presentation. Giving each student in the group the same grade stimulates active involvement in the group process. Students who do not contribute significantly during the group breakout sessions are often pressured by their peers in the group to increase their contribution. During the last hour of recitation, the class is brought back together and a member from each group (chosen at random by the faculty member) presents 1 of the case write-ups to the class and faculty member. Since students do not know ahead of time which of them will be presenting, they all need to be familiar with the content of the final case write-up. Students are encouraged to use PowerPoint slides or overhead transparencies for their presentation. During this presentation phase, input and questions from students and faculty members are likewise encouraged. Presentation of cases in front of students and faculty members also allows students to gain valuable experience in public speaking and case presentation.
Objectives
At the conclusion of the presentation, students should be able to:
* Discuss various factors that contribute to the development of HIV drug resistance.
* Explain the mechanism of action for each class of antiretroviral agents.
* Detail the mechanism of HIV drug resistance to each class of antiretroviral agents.
* Interpret data on the incidence and extent of HIV drug resistance.
* Provide an overview of the utility of HIV drugresistance testing including a contrast between phenotypic and genotypic testing.
* Discuss new classes of anti-HIV drugs and potential new HIV targets that are under investigation.
* Apply the material leaned in this presentation to clinically relevant case studies.
Factors Contributing to the Development of HIV Drug Resistance
Several factors related to the life cycle and replication of HIV are key contributors toward the rapid and widespread emergence of resistance that is seen with this organism. First theHIVreverse transcriptase (RT) enzyme is notoriously ''low fidelity'' (ie, the enzyme is somewhat nonselective during the copying process) and is prone to errors when copying viral RNA into DNA. By some estimates, HIV RT makes one error in each HIV genome per round of replication.4 This translates into roughly 1 mutation for every 2000 nucleosides. While most of these errors are base substitutions, other mutations such as insertions or duplications, can also occur. Second, HIV has an exceptionally high rate of replication; several billion new viral particles may be produced each day in the untreated patient. In most HIV-infected untreated adults, plasma HIV RNA levels range between 103 and 105 copies/ ml, but can be greater than 106 copies/ml in acute infection or with advanced disease. Since the half-life of cells infected with HIV is generally 1-2 days,5 HIV must infect new cells at a very high rate to maintain the infection at a steady level. This high rate of replication coupled with the high rate of error for RT means that numerous HIV ''variants'' are rapidly formed and propagated. Patients who are infected with HIV can have multiple variants of the virus present in their system. These variants can have greatly different sensitivities to antiretroviral agents, a factor that can significantly complicate the selection of drugs and the course of therapy. Additional factors that may contribute to the development of HIV drug resistance include poor patient compliance, subtherapeutic blood levels of antiretroviral agents, and inappropriate choice of antiretroviral agent(s). Patients should be told to take their HIV medications as prescribed and not to miss any doses. Pharmacokinetic factors that can affect blood levels of antiretroviral agents include poor oral absorption and alteration of drug metabolizing enzymes by other agents, as well as various drug-drug interactions.
While some HIV variants may exhibit intrinsic or ''primary'' resistance to antiretroviral agents, most drug resistance develops as a result of exposure to these agents. Antiretroviral resistance can still occur even during successful therapy of HIV infection.6 Any mutations that confer a selective advantage to a particular viral variant will allow that particular viral variant to predominate. In a sense, the very use of antiviral agents exerts a ''selective pressure'' that favors propagation of resistant viruses. The use of multiple drugs in combination is one strategy of reducing the chance that a resistant viral variant will survive treatment.
Mechanism of Action and Resistance for Antiretroviral Drugs
A listing of most commonly used anti-HIV drug classes can be found in Table 1 along with their class, mechanism of action, and major adverse effects. An excellent review of mechanisms of HIV drug resistance can be found in Clavel (2004).7
Anti-HIV Nucleoside and Nucleotide Analogues. HIV belongs to a class of viruses called retroviruses. These organisms are enveloped viruses that have RNA as their genetic material and utilize the enzyme reverse transcriptase. Reverse transcriptase is an RNA-directed DNA polymerase that copies the HIV RNA