Ritonavir does not inhibit calpain in vitro

Area of Science:

• Biochemistry
• Pharmacology
• Molecular Biology

Background:

• Ritonavir is a known inhibitor of Human Immunodeficiency Virus type 1 (HIV-1) protease.
• Previous reports suggested ritonavir may also inhibit calpain, a calcium-dependent cysteine protease.
• Calpains play crucial roles in various cellular processes, making their inhibition a potential therapeutic target.

Purpose of the Study:

• To investigate the direct inhibitory effects of ritonavir on m-calpain and mu-calpain isoforms.
• To determine if ritonavir can block the enzymatic activity of calpains in vitro.
• To clarify the mechanism of potential calpain-related effects of ritonavir in cellular contexts.

Main Methods:

• In vitro enzymatic assays were performed to assess ritonavir's effect on calpain activity.
• Both autolytic and hydrolytic activities of purified m-calpain and mu-calpain were measured.
• Ritonavir's inhibitory capacity against HIV-1 protease was confirmed as a positive control.

Main Results:

• Ritonavir demonstrated no inhibitory effect on the autolytic activity of either m-calpain or mu-calpain.
• Ritonavir also failed to inhibit the hydrolytic activity of both calpain isoforms in vitro.
• Ritonavir retained its potent inhibitory activity against HIV-1 protease, confirming assay validity.

Conclusions:

• Ritonavir does not directly inhibit the enzymatic activity of m-calpain and mu-calpain isoforms.
• Observed cellular effects of ritonavir involving calpains are unlikely to be due to direct protease inhibition.
• Alternative, indirect mechanisms must be responsible for any calpain modulation by ritonavir in cellular systems.