All of the following drugs are protease inhibitors, EXCEPT:

Abacavir is nucleoside reverse transcriptase inhibitor and not a protease inhibitor. Nelfinovir, Titonavir and Saquinavir are all protease inhibitor. Ref: Antiretroviral Therapy By Erik De Clercq, 2001, Page 147 ; Harrison's Principles of Internal Medicine, Pages 1124, 1126.