Proteasome Inhibitors: Bortezomib and Carfilzomib Explained
Peptide-derived drugs
Bortezomib and carfilzomib are proteasome inhibitors, a class of anticancer drugs that grew out of short peptide chemistry: bortezomib is described as a modified dipeptidyl boronic acid derived from leucine and phenylalanine, and carfilzomib is an epoxomicin derivative. This article explains what the proteasome does, how each drug blocks it, how the two bindings differ, and what the labels warn about.
What are proteasome inhibitors?
Cells constantly dispose of proteins they no longer need. Many are first tagged with a chain of ubiquitin and then fed into the proteasome, a large barrel-shaped enzyme complex that cuts them into short peptides. Proteasome inhibitors are drugs that block this disposal step. In a 1999 Cancer Research paper, Adams and colleagues proposed that disrupting the ubiquitin-proteasome pathway, which degrades proteins involved in cell-cycle control and tumour growth, could slow tumours and trigger cell death. Their lead compound, PS-341, later developed as bortezomib, was broadly cytotoxic across human tumour cell lines in the NCI in vitro screen.
Proteasome inhibitors are a good example of why the boundary between “peptide” and “small molecule” is blurry. They are built from amino-acid-like pieces, but they are modified with chemical groups that let them bind an enzyme’s active site; our explainer on peptides versus small-molecule drugs covers that spectrum.
How proteasome inhibitors work
The proteasome’s 20S core contains several protease activities. The chymotrypsin-like activity, carried by the beta-5 subunit, is the one both drugs target. The 2004 EMEA scientific discussion for Velcade describes bortezomib as a selective and reversible inhibitor of 20S proteasome activity that specifically inhibits the chymotrypsin-like activity. The carfilzomib monograph on HemOnc describes it as irreversibly binding and inhibiting the same chymotrypsin-like activity of the 20S core. Blocking the proteasome causes polyubiquitinated proteins to accumulate, which may lead to cell-cycle arrest and apoptosis.
Bortezomib: a modified dipeptide that binds reversibly
The EMEA document gives bortezomib’s chemical description as a modified dipeptidyl boronic acid derived from leucine and phenylalanine, with a pyrazinylcarbonyl group on the phenylalanine end. The boronic acid group is thought to form a stable tetrahedral complex with the N-terminal threonine residue of the 20S beta-5 subunit. The same document reports a dissociation half-life of 20 minutes and a Ki of 0.62 nM, which is why it is classed as reversible but tightly binding.
The original proof-of-concept data came from animal models. In the 1999 paper, weekly intravenous dosing of PS-341 reduced tumour burden by about 60% in mice bearing PC-3 prostate tumours, and the compound spread widely after intravenous dosing but showed no apparent entry into the central nervous system. Those are mouse findings and say nothing about human dosing.
Carfilzomib: an epoxomicin derivative that binds irreversibly
Carfilzomib is described by HemOnc as an epoxomicin derivative with potential antineoplastic activity, given intravenously. Unlike bortezomib, it binds irreversibly, so recovery of proteasome activity after exposure depends on new proteasomes being made. The FDA label (revised 5/2025) lists indications in relapsed or refractory multiple myeloma, in combinations and as a single agent after one or more prior lines.
What the proteasome inhibitor labels warn about
The Kyprolis label’s warnings and precautions include cardiac toxicities, pulmonary toxicity, infusion-related reactions, thrombocytopenia, hepatic toxicity and hepatic failure, thrombotic microangiopathy, posterior reversible encephalopathy syndrome and embryo-fetal toxicity. The 2004 bortezomib discussion reported peripheral neurotoxicity, primarily in sensory neurons, in rats, monkeys and humans, and sensory peripheral neuropathy was dose-limiting in one early phase I study. That older document is a historical source; the current prescribing information is the reference for present safety wording.
Why proteasome inhibitors matter to peptide science
Proteasome inhibitors show how peptide scaffolds are repurposed: a short chain gives selectivity for an enzyme’s substrate pocket, and a reactive group converts a substrate-like molecule into an inhibitor. Related ideas appear in peptide-drug conjugates and in cyclic peptide drugs such as cyclosporine. For a general look at how peptides are broken down, see peptide degradation. Concentration arithmetic for research-grade peptides can be done with the reconstitution calculator.
Frequently asked questions about proteasome inhibitors
Are bortezomib and carfilzomib peptides?
They are peptide-derived. Bortezomib is described as a modified dipeptidyl boronic acid and carfilzomib as an epoxomicin derivative; both are chemically modified rather than natural peptides.
What is the difference between reversible and irreversible inhibition?
A reversible inhibitor such as bortezomib can come off the enzyme (reported dissociation half-life about 20 minutes), whereas an irreversible inhibitor such as carfilzomib binds permanently, so activity returns only as new proteasomes are made.
Which proteasome subunit do they target?
Both target the chymotrypsin-like activity of the 20S core, associated with the beta-5 subunit.
Is the 1999 mouse data relevant to patients?
It established proof of concept in cells and mice. Human efficacy and safety come from later clinical trials and are summarized in the prescribing information, not in that early paper.
References
- Adams J et al. Proteasome inhibitors: a novel class of potent and effective antitumor agents. Cancer Res 1999;59(11):2615-2622.
- EMEA. Velcade (bortezomib) scientific discussion (2004).
- HemOnc.org. Carfilzomib (Kyprolis).
- U.S. FDA. Kyprolis (carfilzomib) prescribing information, revised 5/2025.
Informational only — not medical advice. This article is for educational purposes. It does not recommend, sell or instruct personal human use of any compound. Consult a qualified healthcare professional for any health decision. Content is intended for adults 21+. Verify scientific details against the primary sources cited.