Intracellular carbovir‐ and lamivudine‐triphosphate concentrations in PBMC and DBS among children with HIV‐1 receiving ABC/DTG/3TC: Results from IMPAACT 2019

13 Background: Abacavir (ABC) and lamivudine (3TC) are recommended components of antiretroviral regimens for treatment in infants and children with HIV. ABC and 3TC are converted intracellularly to their active triphosphorylated (TP) forms: carbovir (CBV)-TP and 3TC-TP, respectively. Limited data exist on their cellular pharmacology in paediatric populations, notably red blood cells as measured in dried blood spots (DBS). Intracellular concentrations in DBS and peripheral blood mononuclear cells (PBMC) may provide insight into exposure-response relationships at the site of action and drug adherence, respectively. We describe CBV-TP and 3TC-TP concentrations in DBS and PBMC among children enrolled in the IMPAACT 2019 trial. Materials and methods: IMPAACT 2019 was a Phase I/II study evaluating the pharmacokinetics, safety, tolerability and efficacy of fixed dose combination (FDC) ABC/dolutegravir (DTG)/3TC in children <12 years of age and weighing 6–<40 kg with HIV-1 (NCT03760458). Participants were enrolled across five weight bands (WB, 6–<10 kg, 10–<14 kg, 14–<20 kg, 20–<25 kg, and ≥25–<40 kg). Those weighing 6–<25 kg received 60/5/30 mg dispersible tablets (3–6 tablets based on weight), while those 25–<40 kg received a 600/50/300 mg immediate-release tablet once daily. Participants were ART-naïve or virologically suppressed on stable ART for ≥6 months prior to entry. DBS and PBMC samples were collected at weeks 1, 4, 12, 24, 36 and 48 in all participants and weeks 8, 16 and 20 among those with the documented M184V mutation. CBV-TP and 3TC-TP were quantified using a validated LC–MS/MS assay. Results are reported as either fmol (CBV-TP) or pmol (3TC-TP) per 2 × 7mm punches (DBS) or 106 cells (PBMC); values below the limit of quantification (BLQ) were imputed as half the lower limit of quantification (LLOQ). Linear mixed effects models were performed with log-transformed CBV-TP and 3TC-TP in each cell type to account for repeated measures across study visits and compare concentrations by WB. Geometric means (GM) with 95% confidence intervals (CIs) were calculated and compared to historical data in adults (n = 12) with HIV-1 on ABC/3TC (NCT04301661). Results: A total of 314 dB and 304 PBMC results were available in 55 participants (44% female; 67% Black, 31% Asian; 7% with M184V). CBV-TP and 3TC-TP were quantifiable in all DBS samples; in PBMC, 10 (3.3%) CBV-TP and 6 (2.0%) 3TC-TP were BLQ. The GM (95% CI) for CBV-TP and 3TC-TP in DBS were 583 (526–646) fmol/punches and 8.99 (8.17–9.88) pmol/punches, respectively. The GM (95% CI) for CBV-TP and 3TC-TP in PBMC were 146 (95.6–153) fmol/106 cells and 7.97 (6.88–9.24) pmol/106 cells, respectively. No differences were identified by WB for CBV-TP or 3TC-TP in DBS or PBMC. Compared to historical data in adults, GM CBV-TP was approximately 52% and 86% higher in DBS and PBMC, respectively; 3TC-TP concentrations were comparable (−11% in DBS and +8% in PBMC). Conclusions: CBV-TP and 3TC-TP concentrations were quantifiable and generally consistent across study visits and WBs in both DBS and PBMC in children receiving FDC ABC/DTG/3TC. These results support evaluating DBS utility in paediatric studies and further exploring factors that may influence intracellular concentrations and relationships with adherence.