This study will evaluate the intracellular pharmacokinetics and platelet effects of abacavir (ABC), lamivudine (3TC), tenofovir alafenamide (TAF), and emtricitabine (FTC) in persons living with HIV that are receiving these medications as part of standard HIV care.

Participants remaining on ABC/3TC- or TAF/FTC-containing therapy will be on study for 4 weeks, and will have two visits: a screening visit and one short PK visit consisting of a single blood draw at week 4.

Participants switching from their ABC/3TC-containing therapy will be on study for 3 weeks, and will have nine visits: a screening visit and 8 short PK visits consisting of a single blood draw at Day 0, 1, 3, 7, 10, 14, 18, and 21.

Abacavir and lamivudine are recommended antiretroviral medications used in the treatment of human immunodeficiency virus (HIV) infection in the United States and globally. Both agents are nucleos(t)ide reverse transcriptase inhibitors (NRTIs), which exert their antiviral activity following entry into target cells and phosphorylation by intracellular kinases to their active anabolites, carbovir-triphosphate (CBV-TP) and lamivudine-triphosphate (3TC-TP). There is limited knowledge regarding the pharmacokinetic (PK) disposition of abacavir and lamivudine anabolites in red blood cells (RBCs), neutrophils, and platelets. Abacavir has also been linked with prothrombotic activity and an increased risk of cardiovascular events in patients on this therapy, central theories of which point towards interference with purinergic signaling due to its structural similarity to endogenous adenosine and guanosine. These findings have not been replicated with other NRTI medications, such as tenofovir. This pilot study will characterize the pharmacokinetics (PK) of these medications in different cell types of persons living with HIV (PLWH) on these therapies as part of clinical care, and will examine endogenous nucleotide levels and metabolic profiles through the use of metabolomics in platelets specifically to better understand what changes might be happening within this cell type with the use of these medications.

Inclusion Criteria:

ABC/3TC Cohort:

• On abacavir 600 mg/lamivudine 300 mg-containing regimen as part of their ART for at least 6 months prior to entry
• HIV-1 RNA <200 copies/mL at screening and within the previous 6 months

TAF/FTC Cohort:

• On tenofovir alafenamide 25 mg/emtricitabine 200 mg-containing regimen as part of standard care for at least 6 months prior to entry
• HIV-1 RNA <200 copies/mL at screening and within the previous 6 months

Switch Cohort:

- Switching from an abacavir/lamivudine-containing regimen (to any other ART regimen not containing ABC/3TC) as part of standard care as recommended by their HIV provider

Exclusion Criteria:

• eGFR <50 mL/min/1.73 m2
• Platelet count <100,000 cells/mm3
• Current or previous use (within 30 days) of anticoagulant or antiplatelet medications (e.g., aspirin, P2Y12 inhibitors, vitamin K antagonists, anti-Xa inhibitors, thrombin inhibitors, etc.)
• History of cardiovascular event(s) (e.g., myocardial infarction, cerebrovascular accident (stroke), peripheral arterial thrombosis, etc.), platelet or bleeding disorders
• Pregnant or planning pregnancy
• Any uncontrolled medical, social, or mental-health issue(s) that, in the opinion of the investigators, could interfere with study participation or the study outcomes
• Inability to comply with directly observed dosing (i.e., lack of availability or ability to use video streaming technology)

Persons living with HIV receiving either abacavir/lamivudine or tenofovir alafenamide/emtricitabine as part of standard HIV care, or planning a switch from an abacavir/lamivudine-containing regimen.