Coadministration of prednisolone (20 mg) and ritonavir (200 mg twice daily) increased prednisolone AUC by 28% and Cmin by 9%. Careful monitoring of therapeutic and adverse effects is recommended when prednisolone is concomitantly administered with ritonavir. The AUC of the metabolite prednisolone increased by 37% and 28% after 4 and 14 days ritonavir, respectively.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

The effect of ritonavir (200 mg twice daily) on single doses of prednisone (20 mg) was assessed in HIV-negative subjects after 4 (n=10) and 14 (n=9) days of ritonavir treatment. Systemic exposure of prednisolone, the active metabolite of prednisone, was significantly increased by ritonavir. Significant changes in prednisolone AUC and clearance were noted on both days 4 and 14. Geometric mean prednisolone AUC increased from 2261 ng.h/ml at baseline to 3098 ng.h/ml on day 4 and 2906 ng.h/ml on day 14. Prednisolone clearance decreased from 8.84 L/h at baseline to 6.45 /h on day 4 and 6.88 /h on day 14. The half life of prednisolone increased from 2.96 h to 3.92 h on day 4, though there was no significant difference by day 14 (3.41 h). No significant differences in Cmax and Tmax were noted on either day 4 or 14. Further studies are warranted on the clinical effects of this interaction in HIV-infected subjects and at clinically relevant ritonavir concentrations.
Prednisolone pharmacokinetics in the presence and absence of ritonavir after oral prednisone administration to healthy volunteers. Penzak SR, Formentini E, Alfaro RM, et al. J Acquir Immune Defic Syndr, 2005, 40(5):573-580.