Coadministration of nelfinavir (750 mg three times daily) and ritonavir (500 mg single dose) had a minimal effect (<10%) on ritonavir AUC or Cmax. Coadministration of a single dose of nelfinavir (750 mg) and ritonavir (500 mg twice daily) increased nelfinavir AUC by 152%. Coadministration of nelfinavir (1250 mg twice daily) and ritonavir (100 or 200 mg twice daily) increased the AUC of nelfinavir by 20% (morning dose) and 39% (evening dose). The AUC of the M8 metabolite increased by 74% (morning dose) and 86% (evening dose). There were no significant differences between low doses of ritonavir (either 100 or 200 mg twice daily) for effects on AUCs of nelfinavir and M8. The clinical relevance of these findings has not been established.
Viracept Summary of Product Characteristics, Roche Products Ltd, September 2011.

Concentrations of nelfinavir were increased when coadministered with ritonavir. Coadministration of nelfinavir (750 mg every 8 hours for 5 doses) with ritonavir (500 mg single dose), in 10 subjects, caused no alteration in ritonavir AUC or Cmax. Coadministration of ritonavir (500 mg every 12 hours for 3 doses) with a single dose of nelfinavir (750 mg), in 10 subjects led to a 152% and 44% increase in AUC and Cmax, respectively. Cmin determination was not relevant for single-dose treatment. An appropriate dose of nelfinavir for this combination, with respect to safety and efficacy, has not been established.
Viracept Prescribing Information, Agouron Pharmaceuticals Inc, April 2012.

Coadministration of nelfinavir (1250 mg twice daily) and ritonavir (100 mg twice daily) increased nelfinavir AUC by 20-39%. Coadministration of a single dose of nelfinavir (750 mg) and ritonavir (500 mg twice daily) increased nelfinavir AUC 152%, but had no effect on ritonavir AUC or Cmin. Ritonavir increases the serum levels of nelfinavir as a result of CYP3A4 inhibition. Appropriate doses for this combination, with respect to efficacy and safety, have not been established. Minimal benefit of ritonavir-mediated pharmacokinetic enhancement is achieved with doses higher than 100 mg twice daily.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (400 mg twice daily) and nelfinavir (500 mg or 750 mg twice daily) was studied in 10 HIV patients. Ritonavir increased the AUC of NLF 500 mg (28 µg/ml.h) and 750 mg bd (26 µg/ml.h) when compared with historical 750 mg controls (20 µg/ml.h). AUC of M8 also increased: 19 µg/ml.h (500 mg), 25 µg/ml.h (750 mg), 6 µg/ml.h (750 mg controls). Concurrent ritonavir increased nelfinavir AUC and significantly increased concentrations of M8. Optimal dose of this BD combination must be determined in long-term treatment trials.
Steady-state pharmacokinetic interactions between ritonavir, nelfinavir and the nelfinavir active metabolite M8. Flexner C, Hsu A, Kerr B. 12th World AIDS Conference, 1998, abstract 42265.

This study describes the effect on the pharmacokinetics of nelfinavir (1250 mg bd) of adding 100 or 200 mg RTV. This was a randomised, open-label, one-sequence crossover study in healthy volunteers of NFV dosed alone for 17 days and then with RTV for another 14 days. Irrespective of dose, RTV increased the AUC of NFV by 20% and 39% after both morning and evening administration respectively. The AUC of the metabolite M8 was increased by 74% and 86% after the morning and evening doses (this is either induction of formation of M8 by CYP2C19 or inhibition of the further metabolism of M8 which is catalysed by CYP3A4). During ritonavir combination therapy, a clear but minor drug effect on nelfinavir pharmacokinetics was demonstrated but no dose effect was shown.
Low dose ritonavir moderately enhances nelfinavir exposure. Kurowski M, Kaeser B, Sawyer A, et al. Clin Pharmacol Ther, 2002, 72:123-32.