Coadministration of ketoconazole (200 mg daily) and ritonavir (500 mg twice daily) increased ketoconazole AUC by 3.4-fold and Cmin by 55%. Ritonavir inhibits CYP3A-mediated metabolism of ketoconazole. Due to an increased incidence of gastrointestinal and hepatic adverse events, a dose reduction of ketoconazole should be considered when co-administered with ritonavir dosed as an antiretroviral agent or as a pharmacokinetic enhancer.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

Coadministration of ritonavir (500 mg every 12 hours for 10 days) with ketoconazole (200 mg daily for 7 days), in 12 subjects, caused a 3.4-fold increase in ketoconazole AUC and a 55% increase in Cmax. Ritonavir AUC and Cmax were increased by 18% and 10%, respectively. High doses of ketoconazole (>200 mg/day) are not recommended.
Norvir Prescribing Information, AbbVie Inc, December 2016.

Twelve HIV + individuals received 400 mg twice daily of ritonavir and the hard gel formulation of saquinavir with 200 or 400 mg of ketoconazole. The AUC, Cmax and half-life of ritonavir increased by 29, 62 and 31%, respectively and the CSF concentration increased by 178%. It was concluded that ketoconazole inhibits CYP3A and transport systems such as P-glycoprotein or MRP-1.
Effect of ketoconazole on ritonavir and saquinavir concentrations in plasma and cerebrospinal fluid from patients infected with Human Immunodeficiency Virus. Khaliq Y, Gallicano K, Venance S, et al. Clin Pharmacol Ther, 2000, 68:637–46.

Coadministration of ritonavir (500 mg 12 hourly for 10 days) and ketoconazole (200 mg once daily for 7 days) was investigated in 12 healthy volunteers. There was a 3.4-fold increase in ketoconazole AUC and a 1.6-fold increase in Cmax. Mean half-life increased from 2.7 to 13.2 h. There was an 18% increase in ritonavir AUC and a 10% increase in Cmax in the presence of ketoconazole.
Evaluation of the pharmacokinetics of multiple dose ritonavir and ketoconazole in combination. Bertz R, Wong C, Carothers L, et al. Clin Pharmacol Ther, 1998, 63:230 (abstract PIII-94).