No clinically significant interaction was observed with tenofovir (300 mg once daily) and fosamprenavir/ritonavir (700/100 mg twice daily). No dosage adjustment is necessary.
Telzir Summary of Product Characteristics, ViiV Healthcare UK Ltd, January 2021.

There was no change (10% increase or decrease or less) in plasma amprenavir trough concentrations for subjects receiving tenofovir disoproxil fumarate (300 mg once daily) in combination with fosamprenavir/ritonavir (700/100 mg twice daily, n=45; 1400/200 mg once daily, n=60) when compared with a parallel control group not receiving tenofovir.
Lexiva Prescribing Information, ViiV Healthcare, October 2020.

A higher risk of renal impairment has been reported in patients receiving tenofovir disoproxil fumarate in combination with a ritonavir or cobicistat boosted protease inhibitor. A close monitoring of renal function is required in these patients. In patients with renal risk factors, the co-administration of tenofovir disoproxil fumarate with a boosted protease inhibitor should be carefully evaluated.
Viread Summary of Product Characteristics, Gilead Sciences Ltd, September 2016.

The potential interaction between fosamprenavir and tenofovir disoproxil fumarate was investigated in 36 healthy subjects who received tenofovir (300 mg once daily for 7 days, period 1), and then were randomized to 14 days of either fosamprenavir (1400 mg twice daily) or fosamprenavir/ritonavir (700/100 mg twice daily) alone or with tenofovir (period 2). Subjects continued their randomized dose of fosamprenavir±ritonavir for 14 more days (period 3), adding or removing tenofovir based upon its receipt in period 2. Plasma amprenavir and tenofovir pharmacokinetics were assessed on the last day of each period. Tenofovir Cmin, Cmax and AUC decreased by 12%, 25% and 15% after fosamprenavir coadministration and by 9%, 18% and 7% after fosamprenavir/ritonavir coadministration. Unboosted amprenavir Cmin, Cmax and AUC increased by 31%, 3% and 7% after tenofovir coadministration; boosted amprenavir Cmin, Cmax and AUC increased by 31%, 4% and 16% with tenofovir. No significant changes in ritonavir pharmacokinetics were observed. The authors conclude that the increases in amprenavir exposures and modest decreases in tenofovir exposures are unlikely to be clinically significant.
Steady-state amprenavir and tenofovir pharmacokinetics after coadministration of unboosted or ritonavir-boosted fosamprenavir with tenofovir disoproxil fumarate in healthy volunteers. Luber AD, Condoluci DV, Slowinski PD, et al. HIV Med, 2010,11(3): 193-199.

This study was a prospective phase I crossover study in 30 healthy volunteers who received fosamprenavir/ritonavir (1400/100 or 1400/200 mg once daily) with or without tenofovir (300 mg once daily) for 14 days. Tenofovir was added or discontinued (cross over) for another 14 days, with pharmacokinetics being assessed on days 14 and 28. There was no effect of tenofovir on amprenavir AUC or Cmax in either regimen. Tenofovir led to significant increases in ritonavir AUC (32%) and Cmax (71%) when given with fosamprenavir/ritonavir 1400/200 mg. However, the authors conclude that no dose modification is necessary when combining fosamprenavir/ritonavir with tenofovir.
Fosamprenavir/ritonavir plus tenofovir does not affect amprenavir pharmacokinetics: no effect of tenofovir. Kurowski M, Walli RK, Breske A, et al. AIDS, 2007, 21(10): 1368-1370.

The interaction between TDF (300 mg once daily) and FPV/RTV (1400/100 mg or 1400/200 mg once daily) was investigated in healthy male subjects. The 24 h PK profiles for APV were similar for the two groups. There was no effect of TDF on APV AUC in either the 100 mg (GMR=0.99) or 200 mg (GMR=1.06) RTV groups. APV Cmax was unaffected by TDF in the 100 mg group (GMR=1.04), whereas the 200 mg group showed a non-significant increase (GMR=1.11). APV Ctrough showed non-significant increases in both groups (100 mg GMR=1.24, 200 mg GMR=1.02). There was a non-significant trend to higher RTV AUC in the 200 mg group. TDF monophosphate AUC and Cmax showed a non-significant trend to being lower in the 200 mg group, but with a higher Ctrough. Coadministration of TDF with FPV/RTV was generally well tolerated and the absence of relevant interactions suggests that no dose modification is necessary.
Coadministration of tenofovir 300 mg once daily with fosamprenavir/ritonavir 1400/100 mg once daily or 1400/200 mg once daily does not affect amprenavir pharmacokinetics. Kurowski, M et al. 6th International Workshop on Clinical Pharmacology of HIV Therapy, Quebec, April 2005, abstract 10.

The objective of this study was to evaluate the clinical relevance of the FPV/RTV-TDF interaction in HIV+ patients (n=21) receiving FPV/RTV (700/100 mg twice daily) in combination with TDF (300 mg once daily). Median (range) trough concentrations were 1586 (867-2925) ng/ml for APV, 166 (30-615) ng/ml for RTV and 64 (31-226) ng/ml for TDF. Trough concentrations were in the adequate ranges and no clinically relevant PK interaction was found.
Plasma concentrations of amprenavir, ritonavir and tenofovir in HIV-infected patients treated with fosamprenavir/ritonavir (700/100 mg BID) and tenofovir (300 mg QD) containing regimen. Peytavin, G et al. 6th International Workshop on Clinical Pharmacology of HIV Therapy, Quebec, April 2005, abstract 32.