Medical Pharmacology Chapter 36:  Antiviral Drugs

Protease Inhibitors

• Antiretroviral Drugs Used in Treating HIV Infection

  • →Protease Inhibitors

    • Sites of Antiretroviral Drug Actions⁷

      a: The lifecycle of HIV-1, showing the step at which enfuvirtide acts, and also the steps targeted by other drugs.7  b:  "Simplified schematic of the mechanism by which enfuvirtide inhibits viral entry into host cells.7    The envelope glycoprotein of HIV-1 consists of two non-covalently associated subunits, gp120 and gp41. After attachment of HIV-1 to its target cells carrying the CD4 receptor, gp120 interacts with the CD4 receptor, which initiates a series of conformational changes in gp41 and gp120 that lead to the insertion of a region of gp41 into the membrane of the host cell, and formation of a ' pre-hairpin intermediate' (top). Further changes in the confirmation of gp41 bring the viral and cellular membranes into close enough proximity for membrane fusion (bottom left). Abbreviations: NNRTI, non-nucleoside reverse transcriptase inhibitor; NRTI, nucleoside reverse transcriptase inhibitor; PI, protease inhibitor" Attribution:  Slight modification of Figure 1 from:  LaBonte J Lebbos J Kirkpatrick nature Reviews Drug Discovery 2, 345-346 (May 2003).7

     

    • Introduction:

      • The enzyme HIV protease catalyzes the cleavage of polyproteins into functional structural proteins associated with the mature virion.⁸  

        • In addition HIV protease is an activator of reverse transcriptase and also appears involved in release of infectious virions.

      • The combination of drugs that inhibit the activity of HIV proteases (protease inhibitors, PI) along with reverse transcriptase (RT) inhibitors greatly enhanced HIV treatment outcomes.⁸

      • The HIV protease inhibitors are classified as competitive inhibitors, acting on the virus aspartyl protease.

        • The viral protease consists of two 99-amino acid monomers with each monomer providing an aspartic acid required for catalytic activity.

        • Human aspartyl proteases, including pepsin, renin, cathepsins D and E and gastricsin, by contrast to the viral protease, consist of a single polypeptide chain.

          •  

            Human aspartyl protease:  Renin⁹

            Renin is also known as angiotensinogenase, an important component of the renin-angiotensin-aldosterone system. This system is important in extracellular volume regulation and arterial tone (vasoconstrictive effect). Renin aspartyl protease activity catalyzes hydrolysis of angiotensinogen to angiotensin I.10 Renin-Angiotensin-Aldorsterone System11

        • Furthermore, HIV protease inhibitors do not significantly affect the activity of human aspartyl protease enzymes.

          •  

            HIV-1 Protease¹³

            "HIV-1 Protease has the classic AspTryGly of Aspartyl Proteases. These amino acids are located at position 25,26 and 27 and are responsible for the catalytic activity."14 The proteolytic enzymatic activity appears to occur by a one-step mechanism in which a water molecule serves as nucleophile with an acidic protein serving as an electrophile in peptide bond hydrolysis.15

        • HIV protease is encoded by the pol gene and is expressed as part of the gag-pol polyproteins.^12,3 

          • This gene encodes the 99-amino acid polyprotein noted above.

          • Dimeric forms exhibit aspartyl protease activity; whereas, monomeric forms are catalytically inactive.

        • HIV-1 protease targets particular amino acid sequences in the gag and gag-pol polyproteins.

          •  

            Gag-Pol Polyproteins and Proteolytic Products¹²

            Attribution:  The figure above corresponds to fig 1 of reference 12 (Flexner C HIV-Protease Inhibitors The New England Journal of Medicine 338(18) 1281-1292, April 30, 1998. http://www.ncbi.nlm.nih.gov/pubmed/9562584)

          • These polyproteins must be cleaved for virion maturation.^12,3   

            • Gag polyprotein cleavage results in three large proteins involved in virion structure and RNA packaging.

              • Three smaller proteins are also formed.

              • Mammalian aspartyl proteases are relatively ineffective in cleaving the gag polyprotein.^12,3   

        • HIV protease inhibitors are typically based on amino acid sequences found and recognized and cleaved in HIV proteins. HIV protease inhibitors often are based on synthetic analogues of phenylalanine-proline sequences (positions 167 and 168) of the gag-pol polyproteins.

          • HIV-1 protease inhibitors prevent this critical cleavage of gag and gag-pol polyprotein precursors in both acutely and chronically HIV-1-infected cells.

          • Accordingly, such inhibition prevents additional virion maturation and release, although these drugs would not be effective on cells which are only harboring integrated proviral DNA.^12,3 

    Abbreviations:  Antiretroviral Drugs, Combinations, and Classifications⁵

    3TC = lamivudine	ABC = abacavir	ARV = antiretroviral	ATV/c = cobicistat-boosted atazanavir	ATV/r = ritonavir-boosted atazanavir ear
    DRV/r= ritonavir-boosted darunavir	DTG = dolutegravir	EFV = efavirenz	EVG/c/TDF/FTC = elvitegravir/cobicistat/tenofovir DF/emtricitabine	FTC = emtricitabine
    LPV/r = ritonavir-boosted lopinavir	RAL = raltegravir	RPV =rilpivirine	RTV = ritonavir	TDF = tenofovir disoproxil fumarate
    INSTI = integrase strand transfer inhibitor	NNRTI = nonnucleoside reverse transcriptase inhibitor	NRTI = nucleoside reverse transcriptase inhibitor	PI = protease inhibitor	CrCl = creatinine clearance