Triple NRTI Therapy
There have been reports of a high rate of virological failure and of emergence of resistance at an early stage when abacavir was combined with tenofovir disoproxil fumarate and lamivudine as a once daily regimen.
Ziagen Summary of Product Characteristics, ViiV Healthcare UK Ltd, December 2018.

Triple NRTI Therapy:
There have been reports of a high rate of virological failure and of emergence of resistance at early stage when tenofovir disoproxil fumarate was combined with lamivudine and abacavir as well as with lamivudine and didanosine as a once daily regimen.
Viread Summary of Product Characteristics, Gilead Sciences Ltd, September 2016.

Coadministration of tenofovir-DF (300 mg once daily) and abacavir (300 mg single dose) was studied in 8 HIV-negative subjects. There was no change in tenofovir pharmacokinetic parameters; abacavir AUC was unaltered and Cmax increased by 12%.
Viread Prescribing Information, Gilead Sciences International Inc, February 2016.

The plasma and intracellular pharmacokinetic interaction between tenofovir and abacavir, lamivudine or lopinavir was investigated in HIV+ subjects receiving TDF + 3TC/LPV (n=7), TDF + 3TC/NVP (n=8), TDF + ABC/LPV (n=7) or TDF + ABC/NVP (n=5). Between group comparisons showed no significant interaction between tenofovir and abacavir or lamivudine.
A pharmacokinetic study in HIV infected patients under tenofovir fumarate: investigation of systemic and intracellular interaction between TDF and abacavir, or lamivudine or lopinavir/ritonavir. Pruvost A, et al. 8th International Workshop on Clinical Pharmacology of HIV Therapy, Budapest, April 2007, abstract 56.

The effect of stopping abacavir on the intracellular pharmacokinetics of tenofovir was studied in 7 HIV-infected subjects initially receiving abacavir and tenofovir with lamivudine or stavudine. Intracellular levels of tenofovir-diphosphate did not demonstrate evidence of substantial changes over time after discontinuation of abacavir. Median TDF-DP concentrations prior to and 1 month after discontinuation were 89.9 and 89.6 fmol/10^6 cells, respectively.
Intracellular pharmacokinetics of tenofovir disphosphate, carbovir triphosphate and lamivudine triphosphate in patients receiving triple-nucleoside regimens. Hawkins T, et al. J Acquir Immune Defic Syndr, 2005, 39: 406-411.