Concentrations of tacrolimus and nelfinavir may be increased by coadministration.
Viracept Prescribing Information, Agouron Pharmaceuticals Inc, April 2012.

The interaction between tacrolimus and antiretroviral therapy (ART) was studied in 2 HIV/HCV coinfected patients who had received liver transplants. Tacrolimus pharmacokinetics were determined at steady state during the post operative period before and after the reintroduction of ART once tacrolimus concentrations and liver function had stabilised. Both patients taking nelfinavir required a 75-93% decrease in tacrolimus dose and showed decreases in tacrolimus oral clearance, one from 2137 to 43 ml/min, the other from 257 to 29 ml/min. Concentrations of nelfinavir were within the ranges published for patients with normal liver function tests.
Effect of highly active antiretroviral therapy on tacrolimus pharmacokinetics in hepatitis C virus and HIV co-infected liver transplant recipients in the ANRS HC-08 study. Teicher E, Vincent I, Bonhomme-Faivre L, et al. Clin Pharmacokinet, 2007, 46(11): 941-952.

The management of drug-drug interactions between antiretroviral agents and tacrolimus in 10 HIV patients was reported. Antiretroviral agents were stopped on the day of liver transplantation and reintroduced 10 days later. All patients received tacrolimus and prednisolone as immunosuppressive agents and fluconazole, trimethoprim/sulfamethoxazole, and ganciclovir as primary prophylaxis. Tacrolimus pharmacokinetic parameters were measured 10 days following liver transplantation and 10 days following antiretroviral re-initiation. Tacrolimus target blood concentrations were 8-20 ng/ml from baseline to week 6 and 5-15 ng/ml after week 6; tacrolimus doses were individually adjusted according to the Ctrough measured. Among the patients studied, two were on nelfinavir, three on lopinavir/ritonavir, two on efavirenz and one on three NRTIs. While the NRTIs and efavirenz led to a slight change in tacrolimus pharmacokinetics, nelfinavir and lopinavir/ritonavir caused a large inhibition of tacrolimus metabolism, resulting in an increase in its half-life and a decrease in its clearance. Therefore, tacrolimus plasma concentrations should be monitored and drug dose adjustments performed. No alteration in the antiretroviral pharmacokinetic parameters was observed.
Teicher E, Taburet AM, Vincent I, et al. 12th Conference on Retroviruses and Opportunistic Infections, Boston, February 2005, abstract 662.

There are reports of two cases where blood levels of tacrolimus were elevated when nelfinavir was reintroduced post operatively. In one case, the dose of tacrolimus had to be decreased to 0.5 mg once a week (~1/70 of normal dose) and in the other, coadministration of nelfinavir necessitated a 95% reduction in tacrolimus dose (from 4 mg twice daily to 0.5 mg every 3–5 days). Thus, careful monitoring of drug levels are required and dose reduction should be considered.
Interaction between nelfinavir and tacrolimus after orthoptic liver transplantation in a patient coinfected with HIV and hepatitis C virus. Schvarcz R, Rudbeck G, Soderdhal G & Stahle L. Transplantation, 2000, 69:2194–5.
Concomitant human immunodeficiency virus protease inhibitor therapy markedly reduced tacrolimus metabolism and increased blood levels. Sheikh AM, Wolf DC, Lebovics E, et al. Transplantation, 1999, 68:307–9.