Difelikefalin
CR845, FE 202845, difelikefalin acetate, D-Phe-D-Phe-D-Leu-D-Lys-(4-amino-4-carboxypiperidine)
Difelikefalin is a small synthetic peptide built entirely from D-amino acids that activates kappa opioid receptors but is deliberately engineered not to enter the brain. It is licensed for the severe, relentless itch that affects patients on haemodialysis, a condition with almost no effective treatments. Because it stays outside the central nervous system, it does not produce the dysphoria and hallucinations that made earlier kappa agonists unusable.
Mechanism
Difelikefalin is a selective agonist at the kappa opioid receptor with an important design constraint: peripheral restriction. Its four residues are all D-amino acids (D-Phe-D-Phe-D-Leu-D-Lys) coupled to a 4-amino-4-carboxypiperidine cap. The D-configuration blocks peptidase degradation, while the combination of high hydrophilicity, a permanently charged carboxylate and the peptide backbone gives it essentially no passive membrane permeability, so it does not cross the blood-brain barrier in meaningful quantities. It therefore engages kappa receptors on peripheral sensory neurons and on immune cells (keratinocytes, T lymphocytes, macrophages) while leaving central kappa receptors untouched.
That distinction is the whole point. Kappa agonism has long been known to suppress itch and pain, but centrally acting kappa agonists produce dysphoria, sedation, hallucinations and depersonalisation severe enough to have terminated multiple development programmes. Restricting the drug to the periphery preserves the antipruritic effect and discards the psychotomimetic one.
The pathophysiology it addresses is the itch of chronic kidney disease, which affects a large minority of haemodialysis patients and is associated with poor sleep, depression and increased mortality. One leading hypothesis is an imbalance between mu opioid signalling, which promotes itch, and kappa opioid signalling, which suppresses it, with the balance shifting towards mu in uraemia. Systemic inflammation, peripheral neuropathy and immune dysregulation also contribute. Difelikefalin is thought to act by inhibiting the firing of itch-transmitting peripheral afferents and by dampening pro-inflammatory mediator release from immune cells. Clearance is almost entirely renal, so in dialysis patients (the licensed population) the half-life is greatly prolonged compared with people with normal kidney function.
What the research shows
KALM-1 randomised 378 haemodialysis patients with moderate-to-severe pruritus at 56 US sites to intravenous difelikefalin 0.5 µg/kg or placebo three times weekly for 12 weeks. The primary endpoint (at least a 3-point improvement in the weekly mean Worst Itching Intensity Numerical Rating Scale) was achieved by 82 of 158 patients (51.9%) on difelikefalin versus 51 of 165 (30.9%) on placebo; with multiple imputation the comparison was 49.1% versus 27.9% (p<0.001). Itch-related quality of life measures improved significantly. Diarrhoea, dizziness and vomiting were more common with difelikefalin.
KALM-2 replicated the design in a broader multinational population. A pooled analysis of both trials (Topf and colleagues, 2022; 851 patients randomised, 426 difelikefalin and 425 placebo) confirmed the effect, with a ≥3-point reduction in 51.1% versus 35.2% and a ≥4-point reduction in 38.7% versus 23.4%, and showed the benefit emerged as early as week 1 and was sustained. An independent randomised trial in 178 Japanese haemodialysis patients (Narita and colleagues, 2023) found a week-4 reduction in Worst Itching Intensity of −2.06 points with difelikefalin versus −1.09 with placebo, a between-group difference of −0.97 (p<0.001), with adverse events in 15% versus 3% on placebo, mostly gastrointestinal, a smaller but consistent effect in a different population.
Development has extended to other pruritic conditions. A randomised trial of oral difelikefalin in notalgia paraesthetica, a localised neuropathic itch of the upper back, reported significant reduction in itch intensity versus placebo. Trials in pruritus associated with atopic dermatitis and in chronic liver disease have produced more mixed results. The peripherally restricted kappa agonist concept is under active investigation for pain as well as itch, but outside the haemodialysis indication the evidence remains early.
Evidence assessment
High-quality evidence
Difelikefalin holds MHRA, EMA and FDA marketing authorisations, and NICE recommended it for NHS use in England in 2023. Approval rests on two replicated phase 3 randomised double-blind placebo-controlled trials, KALM-1 and KALM-2, with an independent confirmatory randomised trial conducted in Japan. All three cited papers were verified against PubMed with matching titles, PMIDs, DOIs, journals, years, sample sizes and effect estimates, and the KALM-1 registration NCT03422653 was confirmed on ClinicalTrials.gov. That is replicated randomised evidence plus regulator-approved labelling. The honest qualification concerns effect size: in KALM-1 the responder rate was 51.9% versus 30.9% for placebo, meaning roughly one additional responder for every five patients treated, against a substantial placebo response, a real and clinically useful effect in a condition with few alternatives, not a dramatic one.
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Key studies
A phase 3 trial of difelikefalin in hemodialysis patients with pruritus Preclinical only
At least a 3-point improvement in weekly mean Worst Itching Intensity score was achieved by 82 of 158 difelikefalin patients (51.9%) versus 51 of 165 on placebo (30.9%); with multiple imputation, 49.1% versus 27.9% (p<0.001). Itch-related quality of life improved significantly. Diarrhoea, dizziness and vomiting were more common with difelikefalin.
Efficacy of difelikefalin for the treatment of moderate to severe pruritus in hemodialysis patients: pooled analysis of KALM-1 and KALM-2 phase 3 studies Preclinical only
Difelikefalin showed early (week 1) and sustained efficacy. A ≥3-point reduction in itch severity was achieved by 51.1% versus 35.2% on placebo, and a ≥4-point reduction by 38.7% versus 23.4%. Quality-of-life improvements were greater with difelikefalin across multiple measures, with consistent results across diverse haemodialysis populations.
Difelikefalin for hemodialysis patients with pruritus in Japan Preclinical only
At week 4 the Worst Itching Intensity score fell by 2.06 points with difelikefalin versus 1.09 with placebo, a between-group difference of −0.97 (p<0.001). Quality-of-life measures improved. Adverse events occurred in 15% versus 3% on placebo, predominantly gastrointestinal (constipation, abdominal discomfort). A smaller but directionally consistent effect in an independent population.
Safety
The adverse effect profile is mild and reflects peripheral kappa agonism. Diarrhoea, nausea, vomiting, dizziness, somnolence and mental status changes are the most commonly reported effects; in the Japanese trial, gastrointestinal effects such as constipation and abdominal discomfort dominated. Dizziness and somnolence occur more often than with placebo and are relevant to falls risk in a frail dialysis population.
The crucial safety design feature is what does not happen. Centrally acting kappa opioid agonists cause dysphoria, hallucinations, depersonalisation and sedation severe enough to have ended several drug development programmes. Peripheral restriction largely avoids this, and central nervous system effects in the KALM trials were mild and infrequent. Difelikefalin also does not cause the respiratory depression, euphoria or dependence associated with mu opioid agonists, since it does not act at mu receptors. It is an opioid-receptor drug that is not an opioid in the sense that matters for abuse and overdose. Nonetheless, the labelling advises caution with concomitant sedating agents, including centrally acting opioids, sedating antihistamines and alcohol.
Because elimination is almost entirely renal, difelikefalin accumulates markedly in kidney failure, which is why the licensed population is precisely the population in which the half-life is longest, and why it is given after dialysis in a fixed weight-based bolus into the dialysis circuit. It is not licensed or studied for pruritus outside the haemodialysis setting, and it is a specialist-administered hospital product delivered during dialysis, not a self-administered medicine.
A note on classification: difelikefalin sits at the boundary between peptide and small molecule. At 680 daltons with four D-amino acids and a non-natural piperidine cap, it is often handled as a small-molecule drug in regulatory and pharmacokinetic terms, though its origin and design logic are entirely peptide-based.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Prescription-only medicine (POM). Licensed by the MHRA in 2022 for the treatment of moderate-to-severe pruritus associated with chronic kidney disease in adult haemodialysis patients. Administered as an intravenous bolus into the venous line of the dialysis circuit at the end of the haemodialysis session. Recommended by NICE for NHS use in England in 2023. |
| United States | FDA-approved (August 2021) for the treatment of moderate-to-severe pruritus associated with chronic kidney disease in adults undergoing haemodialysis. Intravenous formulation only; an oral formulation has been studied in other pruritic conditions but is not approved for this indication. |
| WADA (sport) | Not listed on the WADA Prohibited List. The S7 narcotics class is an exhaustive list of specified opioids and does not include difelikefalin; as a peripherally restricted kappa agonist with no central effects and no analgesic use in sport, it falls outside the prohibited categories. Athletes with chronic kidney disease should nonetheless confirm current status with their anti-doping organisation, as the List is revised annually. |
Questions
It acts at an opioid receptor, the kappa subtype, but it is not an opioid in the sense people usually mean. It has no activity at mu receptors, so it does not cause euphoria, respiratory depression, constipation of the opioid type, or dependence, and it has no abuse potential of the kind that defines controlled opioids. It also does not reach the brain in meaningful amounts, so it lacks the dysphoria and hallucinations that centrally acting kappa agonists produce.
Kappa opioid receptors in the brain mediate dysphoria, sedation, depersonalisation and hallucinations, effects so unpleasant that several centrally acting kappa agonists were abandoned in development despite good antipruritic and analgesic activity. Difelikefalin is built from D-amino acids with a charged carboxylate and is highly hydrophilic, so it cannot passively cross the blood-brain barrier. It reaches kappa receptors on peripheral nerves and immune cells and nothing more.
Meaningfully, but not dramatically. In KALM-1, 51.9% of treated patients achieved at least a 3-point improvement in itch score versus 30.9% on placebo, roughly one extra responder for every five treated. The Japanese trial found a difference of about one point on the itch scale. Set against a condition that affects a large minority of dialysis patients, badly disrupts sleep and mood, and has almost no other effective treatments, that is a worthwhile effect, which is why NICE recommended it.
Difelikefalin is cleared almost entirely by the kidneys, so in dialysis patients the half-life stretches to 23-31 hours and the drug is itself removed by dialysis. Administering it as a weight-based bolus into the venous line at the end of a haemodialysis session, three times weekly, matches dosing to the dialysis schedule and means it is given by the dialysis team rather than self-injected. It is not licensed for itch in people who are not on haemodialysis.