Tenofovir DF and Abacavir (Ziagen®, GlaxoSmithKline)
In a PK assessment of the interactions of abacavir and tenofovir in a subset of HIV-infected patients participating in a didanosine (ddI) (Videx®, Bristol-Myers Squibb) drug interaction study, a single dose of abacavir 300 mg was dosed alone and with tenofovir DF 300 mg. Overall, tenofovir DF did not affect abacavir plasma pharmacokinetics clinically nor were there any interactions in vitro. These findings may help to dispel the idea of palliative tenofovir DF/abacavir interactions as the cause of suboptimal clinical outcomes in study 30009 (Kearney BP, et al. Abstracts of the 43rd ICAAC, P. 36:A-1615, 2003).

Tenofovir DF and Lopinavir/Ritonavir (Kaletra®, Abbott)
The pharmacokinetics of lopinavir and ritonavir are unaffected by tenofovir DF, according to a PK study carried out in 27 patients with advanced HIV disease. In contrast, lopinavir/ritonavir increased tenofovir DF concentrations when the two were coadministered. These increases, however, are not clinical significant and have been observed in a similar study. Of added importance, there was little or no clinically significant nephrotoxicity or tenofovir DF discontinuation with long-term use of tenofovir DF and lopinavir/ritonavir in patients with advanced HIV disease. (Kearney BP et al. Abstracts of the 43rd ICAAC, P 37:A-1617, 2003).

Tenofovir DF and Emtricitabine (Emtriva™, Gilead)
In a related assessment of the pharmacokinetic interactions of tenofovir DF and emtricitabine, a newly approved potent NRTI, when the two were co-administered to steady state in 19 healthy volunteers, tenofovir had no effects on the pharmacokinetics of emtricitabine and emtricitabine had no effects on the pharmacokinetics of tenofovir DF. Based on both clinical laboratory and clinical evaluations, tenofovir DF and emtricitabine were well tolerated after multiple-dose administrations alone and together. This should pave the way for the two drugs to be used in a single-tablet formulation, resulting in a lower pill burden and potential better adherence in patients (Blum MR et al. Abstracts of 43rd ICAAC, Pg 38:A1621, 2003).

Tenofovir and Didanosine
While there has been some concern about the use of ddI and tenofovir DF together because of potential pharmaco-kinetic interactions, three month follow-up data from 138 HIV-infected patients in 9 Spanish hospitals demonstrate that whether the combination based on tenofovir DF 300 mg once daily and ddI 400 mg once daily were given to ART-naive patients, used as a simplification regimen, or administered as salvage therapy, HIV RNA levels <=400 copies/mL were seen in 79%, 87%, and 51% of patients respectively. Furthermore, at 6 months, these figures were 80%, 87%, and 66% respectively. Of note, the combination of tenofovir DF plus ddI at full doses did not seem to enhance the risk of toxicity associated with ddI. Also, no kidney dysfunction was seen. (Barrios A, et al. Abstracts of 43rd ICAAC, P.314:H-847, 2003)

Tenofovir DF and Oral Contraceptives
According to a pharmacokinetic and safety study of the co-administration of tenofovir DF and oral contraceptives, the two drugs given together were generally safe and well tolerated. Also, co-administration of norgestimate/ ethinyl estradiol (Ortho Tri-Cyclen®, Ortho-McNeil), a well known oral contraceptive, with tenofovir DF did not affect the pharmacokinetics of either the estrogenic or progestional components, while tenofovir DF pharmacokinetics when given with the oral contraceptive were consistent with previously observed values (Kearney BP et al. Abstracts of the 43rd ICAAC, Pg 37:A1618, 2003).