PROTEASE INHIBITORS (PIs)

PIs target the aspartyl proteinase of HIV, inhibiting the cleavage of viral polyproteins and the subsequent generation of individual viral proteins. This prevents the formation of functional virions, although it does not prevent upstream infection events of parental virions, including viral DNA integration and replication.

At present, there are eight approved PIs (saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir, atazanavir, and fosamprenavir), and several more are under development. Although each is associated with particular side effects to a greater or lesser degree, as a class, their toxicity is relatively homogeneous. Of note, there is a common characteristic of inhibition and/or reduction in cytochrome P-450 enzymes with PI usage, which provides the basis for potentially harmful drug interactions with PIs. However, aside from side effects and potential interactions, there is an accumulating body of evidence, both direct and indirect, to suggest that protease inhibitors may possess antiapoptotic properties apart from their antiviral activity.^42,57-59 Protease inhibitors have been shown to affect a variety of cellular processes, including inhibition of the proteasome,⁶⁰ alteration of antigen presentation and cytotoxic T lymphocyte (CTL) activity,⁶¹ and enhancement of all-trans retinoic acid signaling.⁶² Alteration of lipoprotein metabolism,⁶³ preadipocyte differen­tiation,⁶⁴ and expression of CD36 have also been noted.⁶⁵ Additionally, nelfinavir has been suggested to induce DNA strand cleavage in fully differentiated 3T3-L1 adipocytes,⁶⁶ and apoptotic adipocytes have been observed in fat biopsies taken from protease inhibitor therapy receipients.⁶⁷ Together, this suggests that some combination of these effects may be partially responsible for the lipid abnormalities seen in some patients on PI-based HAART.⁵⁶

Apart from these studies, there have been few specific examinations of protease inhibitors that suggest static or proapoptotic effects.

Additional antineoplastic effects were noted by ritonavir, which decreased endothelial produc­tion of TNF-α, IL-6, IL-8, VEGF, and adhesion molecules and inhibited NF-κB. After treatment with physiologically relevant concentrations, this resulted in apoptosis of Kaposi’s sarcoma derived cell lines in vitro and in tumor progression in mouse models.⁷¹ Finally, it is worth noting that the reported static or apoptotic effects of protease inhibitors seem to affect cancerous cells and, particularly, invasive cells, harder. This allows speculation that such effects, if applicable in vivo, may provide an additional benefit to HIV⁺ individuals with compromised immune systems, in that PI therapy may boost their lymphocyte levels as well as directly prevent or reduce their chances of developing certain types of cancers.

The majority of reports indicate that PIs may directly modulate apoptosis; apart from their antiviral effect that indirectly reduces host cell death, several PIs were shown to possess antiapoptotic biologic activity. Ritonavir can reduce spontaneous apoptosis induced by a variety of stimuli,^75,76 and it was suggested initially that this may be mediated through either a reduction in Fas-L expression by stimulated CD4⁺ cells or a reduction in ICE (caspase-1) expression in both stimulated and unstimulated CD4⁺ cells, irregardless of infection.⁷⁷ There was initially some controversy as to whether the antiapoptotic effects of protease inhibitors were limited to HIV-infected cells^59,78 or whether these effects occurred in both infected and uninfected cells.^76,79-81 However, recent experiments in various sepsis models (another condition involving large amounts of undesired apoptotic cell death) have conclusively demonstrated the ability of protease inhibitors to modulate these processes and protect against cell death.⁸²

The mechanism of this activity is likely to occur at the level of the mitochondria. The inhibition of apoptotic death by nelfinavir was not found to relate to inhibition of caspase-1, -3, -6, -7, or -8 activity, and it was not coincident with changes in the mRNA levels of either pro- or antiapoptotic factors, including caspases, death receptors-4/5, BID, Bfl-1, TRAIL, TNF receptor, XIAP, FAP, cIAP-1/2, Bcl-2, and others.⁷⁶ Instead, protease inhibitors seem to prevent loss of mitochondrial potential even after apoptotic stimuli, suggesting that they have their effect at the level of the permeability transition pore complex (PTPC) (see Figure 25.2). Alternately, there was a role suggested for saquinavir and indinavir in ameliorating oxidative stress; these PIs inhibited apoptosis in normal and transgenic mice expressing HIV genes under basal conditions and under morphine-stimulated states of oxidative stress, in dose-dependent fashion.⁸³ The inhibition of superoxide production conceivably suggests that oxidative stress mitigation is another function of PIs.