Protease inhibitors

Protease inhibitors (PIs) are a diverse class of medications that inhibit the enzymatic activity of proteases—enzymes that break down proteins by hydrolyzing peptide bonds. Proteases play critical roles in many physiological processes, including digestion, blood clotting, immune response, and pathogen replication. In pharmacology, protease inhibitors are primarily recognized for their role in antiviral therapy, particularly in HIV/AIDS and Hepatitis C Virus (HCV) treatment, as well as oncology and anti-inflammatory indications.

The most clinically relevant subclasses include:

1. HIV Protease Inhibitors

2. HCV NS3/4A Protease Inhibitors

3. Investigational and Oncology Protease Inhibitors

This comprehensive review will detail each group’s mechanism, drug profiles, and clinical considerations.

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1. Mechanism of Action

Protease inhibitors bind to the active site of target proteases, preventing the enzyme from cleaving its substrate protein. The nature of inhibition can be:

• Reversible or irreversible

• Competitive or noncompetitive

• Covalent or noncovalent

In HIV:

HIV protease (an aspartyl protease) is essential for post-translational processing of the gag and gag-pol polyproteins into functional viral proteins. Inhibition leads to:

• Production of immature, non-infectious virions

• Reduction in viral load and delayed disease progression

In HCV:

NS3/4A is a serine protease responsible for cleaving the viral polyprotein, essential for replication. Inhibiting it blocks viral replication.

In Cancer:

Some tumors exploit proteolytic enzymes like cathepsins and matrix metalloproteinases (MMPs) for metastasis and invasion. Targeting them can suppress tumor progression.

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2. Classes and Representative Drugs

A. HIV Protease Inhibitors

Approved by the FDA for antiretroviral therapy (ART):

Generic Name	Brand Name
Atazanavir	Reyataz
Darunavir	Prezista
Lopinavir/ritonavir	Kaletra
Ritonavir (booster)	Norvir
Saquinavir	Invirase
Indinavir	Crixivan
Nelfinavir	Viracept
Fosamprenavir	Lexiva
Tipranavir	Aptivus

Note: Ritonavir is often used at low doses as a pharmacokinetic booster rather than a direct antiviral.

B. HCV NS3/4A Protease Inhibitors

Used in Direct-Acting Antiviral (DAA) regimens:

Generic Name	Brand Name	Combination Product
Grazoprevir	—	Elbasvir/grazoprevir (Zepatier)
Simeprevir	Olysio	Obsolete as monotherapy
Paritaprevir	—	Viekira Pak (with ritonavir)
Glecaprevir	—	Glecaprevir/pibrentasvir (Mavyret)
Voxilaprevir	—	Sofosbuvir/velpatasvir/voxilaprevir (Vosevi)

HCV protease inhibitors are now used in fixed-dose combinations with other antivirals (e.g., NS5A/NS5B inhibitors).

C. Investigational and Oncology PIs

• Bortezomib – 26S proteasome inhibitor (approved in multiple myeloma)

• Carfilzomib, Ixazomib – 2nd-gen proteasome inhibitors

• Marizomib – under clinical trials for glioblastoma

• Beloranib – a MetAP2 inhibitor investigated for obesity

• Matrix metalloproteinase inhibitors (MMPIs) – under study for cancer/metastasis

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3. Pharmacokinetics (HIV PIs)

Drug	Half-Life	Metabolism	CYP Inhibition	Food Effect
Atazanavir	~7 h	CYP3A4	Inhibits UGT1A1	Requires food
Darunavir	~15 h	CYP3A4	Inhibits CYP3A4	Requires ritonavir/cobicistat
Lopinavir	~5-6 h	CYP3A	Inhibits CYP3A	Take with food
Ritonavir	~3-5 h	CYP3A, 2D6	Potent CYP3A4 inhibitor	Enhances other PIs

All PIs are metabolized hepatically, and nearly all are substrates and inhibitors of CYP3A4, causing significant drug–drug interaction potential.

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4. Therapeutic Indications

A. HIV Infection

• Combination ART (cART) with 2 NRTIs + 1 PI (boosted)

• Alternative to NNRTI- or integrase-based regimens in select patients

B. Hepatitis C Virus (HCV)

• Fixed DAA combinations (e.g., Mavyret, Zepatier, Vosevi)

• Pan-genotypic or genotype-specific treatment

C. Cancer (Oncology)

• Multiple Myeloma: Bortezomib, Carfilzomib

• Clinical trials in solid tumors

D. Off-label/Investigational

• COVID-19 (lopinavir/ritonavir early in pandemic – ultimately ineffective)

• Anti-inflammatory/autoimmune (via cathepsin inhibition)

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5. Dosing Guidelines (HIV PIs)

Drug	Dose (Adult)	Frequency	Comments
Atazanavir	300 mg + ritonavir 100 mg	Once daily	Avoid with PPIs
Darunavir	800 mg + ritonavir 100 mg	Once daily	Sulfonamide structure
Lopinavir/r	400/100 mg	Twice daily	Fixed-dose combo
Ritonavir (booster)	100 mg	Once/twice daily	Not used alone as ART

6. Adverse Effects

A. HIV PIs

Systemic Area	Adverse Effects
Metabolic	Lipodystrophy, hyperlipidemia, insulin resistance
GI	Nausea, diarrhea, abdominal pain
Hepatic	Transaminase elevation, hepatitis
Renal	Nephrolithiasis (indinavir)
Dermatologic	Rash (sulfa allergy – darunavir, tipranavir)
Cardiac	QT prolongation (saquinavir, ritonavir)

B. HCV PIs

• Elevated bilirubin

• Rash

• Anemia

• Photosensitivity (simeprevir)

C. Proteasome Inhibitors

• Peripheral neuropathy

• Myelosuppression

• Cardiomyopathy

• GI toxicity

• Tumor lysis syndrome

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7. Contraindications

Condition	Specific Agent
Severe hepatic impairment	All PIs (especially darunavir)
Concomitant CYP3A4 drugs	Midazolam, rifampin, simvastatin
Cardiac conditions	QT prolongation risk
Sulfa allergy	Darunavir, fosamprenavir

8. Drug Interactions (HIV PIs)

Due to potent CYP3A4 inhibition, multiple interactions:

Drug/Class	Interaction	Clinical Action
Rifampin	↓ PI levels (induction)	Avoid
Statins (simvastatin)	↑ toxicity (rhabdomyolysis)	Use pravastatin or pitavastatin
Benzodiazepines	↑ sedation/coma (midazolam, triazolam)	Avoid
Oral contraceptives	↓ efficacy	Use alternative contraception
Antacids, PPIs	↓ absorption of atazanavir	Separate dosing or avoid

9. Monitoring Parameters

• Liver function tests (LFTs)

• Lipid panel and glucose (metabolic complications)

• CD4 count and HIV RNA viral load

• Renal function (especially with indinavir)

• ECG (QT prolongation)

• Signs of peripheral neuropathy (oncology use)

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10. Resistance Patterns (HIV)

HIV can develop mutations in the protease gene (e.g., D30N, I50L, V82A), leading to:

• Reduced drug binding

• Cross-resistance across multiple PIs

Boosted regimens (with ritonavir/cobicistat) help maintain higher concentrations to suppress resistant variants.

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11. Pregnancy and Breastfeeding

• Darunavir/ritonavir is preferred in pregnancy

• Atazanavir/ritonavir is an alternative

• Most PIs are compatible with pregnancy, but require monitoring for hepatic function

• Breastfeeding is not recommended in HIV-positive mothers (risk of transmission)

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12. Fixed-Dose Combinations

To improve adherence and reduce pill burden:

• Kaletra: Lopinavir/ritonavir

• Prezcobix: Darunavir + cobicistat

• Evotaz: Atazanavir + cobicistat

Boosters like ritonavir and cobicistat have no antiviral activity but increase PI exposure by CYP3A4 inhibition.

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13. Clinical Pearls

• Boosted PIs are associated with metabolic side effects and drug interactions.

• Atazanavir may cause benign hyperbilirubinemia via UGT1A1 inhibition.

• Ritonavir at low doses is used exclusively as a booster.

• Protease inhibitors are not recommended as monotherapy due to high resistance risk.

• HCV PIs should never be used alone; always in combination regimens.

• Proteasome inhibitors (oncology) require pre-medications to avoid infusion reactions.