carfilzomib
PR-171, PR171, tetrapeptide epoxyketone proteasome inhibitor
Carfilzomib is a second-generation proteasome inhibitor built from a tetrapeptide backbone capped with an epoxyketone warhead, derived from the natural product epoxomicin. Unlike bortezomib it binds the proteasome irreversibly and with far greater selectivity, which spares patients most of the peripheral neuropathy. The trade-off is a distinct and clinically important cardiovascular toxicity profile.
Mechanism
Carfilzomib is a selective, irreversible inhibitor of the chymotrypsin-like activity of the 20S proteasome, acting principally at the β5 subunit of the constitutive proteasome and the LMP7/β5i subunit of the immunoproteasome. Its warhead is an α',β'-epoxyketone, a chemotype found in the Actinomycete natural product epoxomicin. The mechanism of that warhead is elegant and explains the drug's selectivity: the ketone carbonyl is attacked by the hydroxyl of the proteasome's N-terminal threonine (Thr1) while the free α-amino group of the same threonine opens the epoxide, so the two reactions together forge a stable six-membered morpholine ring. Only N-terminal nucleophile hydrolases present both a hydroxyl and a free α-amine in the correct geometry, so epoxyketones essentially cannot react with serine or cysteine proteases. This is why carfilzomib has little of the off-target protease inhibition attributed to bortezomib's boronate, and why treatment-emergent peripheral neuropathy rates are low.
The tetrapeptide portion (a morpholinoacetyl cap on homophenylalanine, followed by leucine, phenylalanine and a leucine-derived epoxyketone) provides the substrate-mimetic recognition that docks the molecule into the S1 to S4 pockets. Because binding is covalent and irreversible, proteasome activity can only be restored by de novo synthesis of new proteasome subunits, which takes roughly 24 hours. This is the reason a drug with a plasma half-life under an hour can be given on an intermittent schedule and still produce sustained, deep proteasome inhibition. The downstream biology mirrors bortezomib: accumulation of polyubiquitinated proteins, terminal unfolded protein response, NF-κB blockade through IκBα stabilisation, accumulation of NOXA, p53 and p27, and apoptosis in cells with a high secretory burden.
What the research shows
The single-arm PX-171-003-A1 phase 2 study in 266 patients with relapsed and refractory myeloma, most of whom were refractory to both bortezomib and lenalidomide, reported an overall response rate of 23.7% with a median duration of response of 7.8 months and median overall survival of 15.6 months, modest numbers, but in a population with essentially no remaining options, and enough for accelerated approval. The ASPIRE phase 3 trial then randomised 792 patients with one to three prior therapies to carfilzomib plus lenalidomide and dexamethasone or to lenalidomide and dexamethasone alone: median progression-free survival was 26.3 versus 17.6 months (HR 0.69), response rate 87.1% versus 66.7%, and the final analysis showed median overall survival of 48.3 versus 40.4 months (HR 0.79). ENDEAVOR compared carfilzomib with bortezomib head to head, each with dexamethasone, in 929 patients; median progression-free survival was 18.7 versus 9.4 months (HR 0.53), and the pre-specified interim overall survival analysis gave 47.6 versus 40.0 months (HR 0.79) in favour of carfilzomib. This is one of the few instances in myeloma where one proteasome inhibitor has been shown directly superior to another.
The cardiovascular signal is the counterweight and it emerged clearly from the same trials. In ENDEAVOR, grade 3 or worse hypertension occurred in 9% of carfilzomib patients versus 3% with bortezomib, dyspnoea in 5% versus 2%, and cardiac failure in 5% versus 2%. Meta-analyses across the carfilzomib programme have put the incidence of any-grade cardiovascular adverse events at roughly 18% and grade 3 or worse at around 8%. Peripheral neuropathy, by contrast, was far less common than with bortezomib (grade 2 or worse 6% versus 32% in ENDEAVOR), confirming the prediction from the epoxyketone chemistry. Not every carfilzomib trial has been positive, and the largest negative one matters: CLARION randomised 955 newly diagnosed transplant-ineligible patients to carfilzomib or bortezomib with melphalan and prednisone and found median progression-free survival of 22.3 versus 22.1 months (HR 0.906, p = 0.159), with no survival advantage. Carfilzomib has not established a role in first-line therapy in that population.
Evidence assessment
High-quality evidence
Regulator-approved labelling in the US, EU and UK supported by two large randomised phase 3 trials (ASPIRE and ENDEAVOR), both with mature overall survival analyses demonstrating benefit, plus the single-arm phase 2 study that supported accelerated approval. All five cited trials plus the negative CLARION study were individually verified against PubMed. The tier reflects that base; CLARION's failure in the newly diagnosed transplant-ineligible setting is reported alongside it rather than glossed over.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
A phase 2 study of single-agent carfilzomib (PX-171-003-A1) in patients with relapsed and refractory multiple myeloma Preclinical only
Overall response rate 23.7%, median duration of response 7.8 months, median overall survival 15.6 months. Peripheral neuropathy was infrequent despite prior bortezomib exposure. Basis for FDA accelerated approval.
Carfilzomib, lenalidomide, and dexamethasone for relapsed multiple myeloma Preclinical only
Median progression-free survival 26.3 versus 17.6 months (HR 0.69) and overall response rate 87.1% versus 66.7% when carfilzomib was added to lenalidomide and dexamethasone.
Carfilzomib and dexamethasone versus bortezomib and dexamethasone for patients with relapsed or refractory multiple myeloma (ENDEAVOR): a randomised, phase 3, open-label, multicentre study Preclinical only
Median progression-free survival 18.7 versus 9.4 months favouring carfilzomib (HR 0.53). Grade 2 or worse peripheral neuropathy 6% versus 32%, but grade 3 or worse hypertension 9% versus 3% and cardiac failure 5% versus 2%.
Carfilzomib or bortezomib in relapsed or refractory multiple myeloma (ENDEAVOR): an interim overall survival analysis of an open-label, randomised, phase 3 trial Preclinical only
Median overall survival 47.6 versus 40.0 months (HR 0.79), the first demonstration that one proteasome inhibitor prolongs survival compared with another in myeloma.
Improvement in overall survival with carfilzomib, lenalidomide, and dexamethasone in patients with relapsed or refractory multiple myeloma Preclinical only
Median overall survival 48.3 versus 40.4 months (HR 0.79), confirming a durable survival advantage for the carfilzomib triplet.
Carfilzomib or bortezomib with melphalan-prednisone for transplant-ineligible patients with newly diagnosed multiple myeloma Preclinical only
Negative trial. Median progression-free survival 22.3 versus 22.1 months (HR 0.906, 95% CI 0.746-1.101, p = 0.159), with no overall survival advantage. Carfilzomib was not superior to bortezomib in the first-line transplant-ineligible setting.
Safety
Cardiovascular toxicity is the defining concern: new or worsening hypertension, heart failure with reduced ejection fraction, ischaemic events, arrhythmias and pulmonary hypertension. Baseline cardiac assessment, blood pressure optimisation and careful fluid management are routine, and older patients and those with pre-existing cardiac disease are at highest risk. Infusion-related reactions (fever, rigors, dyspnoea, flushing) occur particularly with the first doses and are mitigated by dexamethasone pre-medication. Thrombotic microangiopathy, including thrombotic thrombocytopenic purpura and haemolytic uraemic syndrome, has been reported and requires immediate discontinuation. Acute kidney injury, tumour lysis syndrome, hepatic failure, posterior reversible encephalopathy syndrome, venous thromboembolism, cytopenias, and reactivation of hepatitis B and herpes zoster all appear in the labelling. Interstitial lung disease and pulmonary hypertension are uncommon but serious. Peripheral neuropathy is notably less frequent and less severe than with bortezomib.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | EU marketing authorisation granted November 2015 for adults with multiple myeloma who have received at least one prior therapy, in combination regimens; licensed in Great Britain by the MHRA. Recommended by NICE in defined combination settings and treatment lines. Current NICE guidance should be checked for the applicable appraisal. Prescription-only, hospital-administered. |
| United States | FDA accelerated approval July 2012 as monotherapy for relapsed and refractory multiple myeloma after at least two prior therapies including bortezomib and an immunomodulatory agent. Regular approval July 2015 in combination with lenalidomide and dexamethasone; extended January 2016 in combination with dexamethasone alone, and subsequently in combination with daratumumab (2020) or isatuximab (2021) and dexamethasone. Intravenous, specialist use only. |
| WADA (sport) | Not on the WADA Prohibited List. Carfilzomib has no anabolic, hormonal or masking activity and falls outside all prohibited classes. |
Questions
It comes down to the chemistry of the warhead. Carfilzomib's epoxyketone needs a target that offers both a hydroxyl group and a free α-amino group in precise geometry, a combination essentially unique to the proteasome's N-terminal threonine. Bortezomib's boronic acid is less discriminating and also inhibits several serine proteases thought to contribute to neuropathy.
Because the binding is irreversible. Once carfilzomib forms its covalent morpholine adduct with the proteasome, that proteasome is permanently disabled. Activity only returns as the cell manufactures replacement subunits, which takes about a day. Plasma half-life is therefore a poor guide to duration of effect.
Cardiovascular toxicity: hypertension, heart failure, ischaemic events and pulmonary hypertension. In the ENDEAVOR trial, grade 3 or worse cardiac failure occurred in 5% of carfilzomib patients versus 2% with bortezomib. Baseline cardiac assessment and close blood pressure monitoring are standard practice.
In the relapsed setting, ENDEAVOR showed it superior for both progression-free and overall survival. That comparison has been criticised on two grounds: the carfilzomib arm used a higher-than-registered dose, and the bortezomib comparator was given intravenously in most patients even though subcutaneous dosing had already become standard and is less neurotoxic. In newly diagnosed transplant-ineligible patients the picture is different again. The CLARION trial in 955 patients found no advantage for carfilzomib. Neither drug wins everywhere.
It is a synthetic analogue of epoxomicin, a natural product isolated from an Actinomycete bacterium and originally noted for antitumour activity. The epoxyketone warhead responsible for that activity was identified as a proteasome-specific inhibitor and then optimised into carfilzomib.