Difelikefalin: Korsuva, FDA-approved anti-itch drug for kidney dialysis patients
A synthetic drug injected at the end of each dialysis session that relieves severe itching in people with chronic kidney disease; FDA-approved in 2021 and, despite acting on opioid receptors, is not a controlled substance.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
Difelikefalin (sold as Korsuva in the US and Kapruvia in the EU; development code CR845) is a synthetic anti-itch drug given as an intravenous injection at the end of each hemodialysis session. It is approved for adults with chronic kidney disease (CKD) who suffer from intense, persistent itching while on hemodialysis — a condition called CKD-associated pruritus that affects a large fraction of dialysis patients and is linked to poor sleep, worse quality of life, and shorter survival (Saucereau and colleagues 2026, Renal Failure). It is the first systemic therapy specifically approved for this indication (Wala and colleagues 2022, Pharmaceuticals; Fugal and colleagues 2023, Annals of Pharmacotherapy). Structurally it is a synthetic all-D-amino-acid tetrapeptide — every residue is the mirror-image (D-configuration) form rather than the standard L-form used by the body, which both prevents the drug from crossing the blood-brain barrier and protects it from proteolytic enzymes; the stored sequence FFFLK uses standard single-letter notation but does not capture this D-configuration or the C-terminal substituent of the active drug.
History
Difelikefalin emerged from a targeted drug-discovery effort at Cara Therapeutics, founded in 2004, to develop kappa opioid receptor (KOR) agonists that act outside the central nervous system — a long-standing pharmacological challenge, since classical kappa opioids produce dysphoria, sedation, and hallucinations through brain-based receptor activation (Fugal and colleagues 2023, Annals of Pharmacotherapy; Lipman and colleagues 2021, Expert Opinion on Pharmacotherapy). The compound was designed as a synthetic D-amino-acid peptide whose hydrophilicity and mirror-image amino-acid composition would prevent meaningful blood-brain-barrier crossing. Early clinical development explored postoperative pain, but the program eventually crystallized around CKD-associated pruritus, which had no approved systemic therapy. The pivotal Phase 3 KALM-1 trial — published in the New England Journal of Medicine in 2020 — enrolled hemodialysis patients with moderate-to-severe pruritus across the US, Canada, and Europe, and demonstrated significant reduction in itch intensity versus placebo (Fishbane and colleagues 2020, NEJM). KALM-2, an international companion trial, supported these findings. On the basis of this program, the FDA approved Korsuva in August 2021, and the EMA approved Kapruvia in April 2022 through a partnership with Vifor Pharma (Fugal and colleagues 2023, Annals of Pharmacotherapy). Subsequent work has characterized pharmacokinetics in renal impairment, abuse-liability and physical-dependence profiles, and effects on sleep quality in hemodialysis patients (Spencer and colleagues 2024, BMC Nephrology; Shram and colleagues 2022, Clinical and Translational Science; Weiner and colleagues 2024, Nephrology Dialysis Transplantation).
What it does
Difelikefalin reduces itching intensity and improves itch-related quality of life and sleep in adults with CKD who are on hemodialysis (Fishbane and colleagues 2020, NEJM; Weiner and colleagues 2024, Nephrology Dialysis Transplantation). It works by activating itch-suppressing kappa opioid receptors on sensory neurons and immune cells in peripheral tissue, without reaching the brain (Wala and colleagues 2022, Pharmaceuticals; Cai and colleagues 2024, Frontiers in Pharmacology). Because the drug is confined to the peripheral nervous system and peripheral immune cells, it does not produce the mood disruption, sedation, or central dependence associated with classical opioid medications. Administration in the approved indication is intravenous via the dialysis circuit by healthcare staff at the end of each hemodialysis session; patients do not self-administer.
Evidence
- Human: Strong. The Phase 3 KALM-1 RCT (Fishbane and colleagues 2020, NEJM) demonstrated that significantly more difelikefalin-treated hemodialysis patients achieved a ≥4-point reduction in Worst Itch Numerical Rating Scale (WI-NRS) score at week 12 versus placebo, with response rates of approximately 49–54% (difelikefalin) versus 28–42% (placebo); itch-related quality-of-life measures (5-D itch scale, Skindex-10) also improved. KALM-2 supported the same population (Rastogi and colleagues 2023, Expert Review of Clinical Pharmacology). A Japanese Phase 3 RCT and a JAMA Network Open publication confirmed efficacy in Japanese hemodialysis patients (Narita and colleagues 2023, NEJM Evidence; Narita and colleagues 2022, JAMA Network Open). A post-hoc analysis of KALM-1 and KALM-2 in Black/African American patients found results consistent with the overall population (Fishbane and colleagues 2024, American Journal of Nephrology). Exploratory analyses of the KALM and Study 3105 datasets characterized outcomes by baseline itch severity and in patients on concomitant opioids (McCafferty and colleagues 2025, Journal of Renal Care; Burton and colleagues 2025, Kidney and Blood Pressure Research). A single-arm Phase 3 trial supported health-related quality-of-life gains (Fotheringham and colleagues 2024, The Patient). Multiple systematic reviews and meta-analyses across published RCTs have confirmed the efficacy and tolerability profile (Zhu and colleagues 2026, Blood Purification; Cai and colleagues 2024, Frontiers in Pharmacology; Saeed and colleagues 2024, Renal Failure; Xue and colleagues 2024, Clinical Nephrology). Phase 2 evidence exists for notalgia paresthetica (Kim and colleagues 2023, NEJM) and for oral difelikefalin in non-dialysis CKD pruritus, but neither use is approved. Difelikefalin has not demonstrated general analgesic efficacy in pain populations; earlier development in postoperative pain did not produce an approved indication.
- Pharmacokinetic studies: Spencer and colleagues (2024, BMC Nephrology) characterized the pharmacokinetic profile across varying degrees of renal impairment. A radiolabeled excretion study in healthy subjects and hemodialysis patients further characterized metabolism and excretion (Clinical Pharmacokinetics 2023).
- Animal: Peripheral kappa opioid pharmacology is well-characterized in animal models and supports the peripheral-restriction concept (Wala and colleagues 2022, Pharmaceuticals).
- In vitro: Selective KOR agonism, inhibition of TRPV1 activity, and reduced substance P release from peripheral nerve terminals are documented at the molecular level (Wala and colleagues 2022, Pharmaceuticals; Rastogi and colleagues 2023, Expert Review of Clinical Pharmacology).
Known effects
- Reduction of CKD-associated pruritus intensity in hemodialysis — FDA-approved (Phase 3 KALM-1, KALM-2)
- Improvement in itch-related quality of life — Phase 3 (Fishbane and colleagues 2020, NEJM)
- Improvement in itch-related sleep disruption — Post-hoc analysis of Phase 3 data (Weiner and colleagues 2024, Nephrology Dialysis Transplantation)
- Improvement in health-related quality of life — Phase 3 single-arm trial (Fotheringham and colleagues 2024, The Patient)
- No abuse-liability signal — Dedicated human pharmacology RCT in recreational polydrug users (Shram and colleagues 2022, Clinical and Translational Science)
- No physical dependence at studied duration — Dedicated RCT in hemodialysis patients (Clinical and Translational Science, doi 10.1111/cts.13538)
- No clinically significant respiratory depression at approved doses — Dedicated RCT in healthy volunteers (Viscusi and colleagues 2021, Clinical and Translational Science)
- Notalgia paresthetica — Phase 2 evidence; not approved (Kim and colleagues 2023, NEJM)
- General analgesia — Not established; earlier development did not yield an approved pain indication
Safety signals
Across the KALM Phase 3 trials, the most common adverse events were dizziness, somnolence, diarrhea, nausea, and hyperkalemia (Fishbane and colleagues 2020, NEJM; Fugal and colleagues 2023, Annals of Pharmacotherapy; Mahmoud and colleagues 2024, Expert Review of Clinical Immunology). The mechanism of dizziness and somnolence is not definitively established — possibilities raised in the literature include residual CNS exposure, peripheral-vagal autonomic effects, or other pathways (Rastogi and colleagues 2023, Expert Review of Clinical Pharmacology). Although difelikefalin has minimal CNS penetration, these effects may be additive with concurrent CNS depressants; exploratory KALM analyses of patients on concomitant opioids inform clinical decision-making (Burton and colleagues 2025, Kidney and Blood Pressure Research). Hyperkalemia is a clinically relevant signal in the CKD population. Dedicated human pharmacology studies established that difelikefalin does not produce clinically significant respiratory depression (Viscusi and colleagues 2021, Clinical and Translational Science) and does not carry abuse liability (Shram and colleagues 2022, Clinical and Translational Science) — findings that supported the FDA scheduling assessment placing it outside the Controlled Substances Act. No physical dependence was observed in the dedicated dependence RCT at the studied protocol duration; long-term safety beyond approximately two years of open-label extension data remains an active pharmacovigilance question. Safety and efficacy in pediatric patients and in pregnancy and breastfeeding are not adequately established.
Regulatory status
- US (FDA): Prescription-only. Korsuva approved August 2021 for moderate-to-severe pruritus associated with CKD in adults undergoing hemodialysis. Not a controlled substance. Distributed through specialty channels integrated with dialysis-center workflows; not available at standard retail pharmacies.
- EU (EMA): Kapruvia approved April 2022 for the same hemodialysis-pruritus indication; marketed through Vifor Pharma partnership.
- UK (MHRA), Canada (Health Canada), Australia (TGA): Approvals obtained per available sources; individual indication details not separately extracted in this card.
- WADA: Not specifically named on the WADA Prohibited List per available sources; difelikefalin lacks central analgesic or performance-enhancing effects relevant to athletes with normal physiology. Status should be confirmed against the current WADA list for any specific therapeutic-use context.
Mechanism
Difelikefalin is a selective agonist at the kappa opioid receptor (KOR). Its all-D-amino-acid tetrapeptide structure and high hydrophilicity prevent it from crossing the blood-brain barrier at clinically relevant concentrations, confining pharmacological activity to peripheral tissues (Cai and colleagues 2024, Frontiers in Pharmacology; Viscusi and colleagues 2021, Clinical and Translational Science). KOR activation on peripheral sensory neurons — including dorsal root ganglion neurons and small-fiber afferents — and on immune cells modulates itch signaling at the periphery rather than suppressing central perception (Wala and colleagues 2022, Pharmaceuticals). At the molecular level, peripheral KOR activation inhibits TRPV1 activity and reduces substance P release from peripheral nerve terminals, attenuating pro-pruritic and pro-inflammatory signaling (Wala and colleagues 2022, Pharmaceuticals; Rastogi and colleagues 2023, Expert Review of Clinical Pharmacology). Because no meaningful KOR activation occurs in the central nervous system, the dysphoria, sedation, and hallucinations characteristic of centrally acting kappa opioids are avoided (Shram and colleagues 2022, Clinical and Translational Science; Viscusi and colleagues 2021, Clinical and Translational Science). The all-D-amino-acid composition also confers protease resistance, supporting pharmacokinetic stability during circulation and dialysis-circuit exposure. Plasma half-life in hemodialysis patients is approximately 2–3 hours per bolus dose; difelikefalin is partially cleared by dialysis itself, and elimination is dominated by renal and dialytic routes with minimal CYP-mediated metabolism (Spencer and colleagues 2024, BMC Nephrology).
Myths and misconceptions
- "Difelikefalin is addictive because it activates opioid receptors." — Difelikefalin activates kappa opioid receptors peripherally and does not meaningfully cross the blood-brain barrier. It does not produce the central reinforcement or euphoria associated with mu-opioid analgesics. Dedicated abuse-liability and physical-dependence studies supported the non-addictive pharmacology, and the FDA scheduling assessment did not place it under Controlled Substances Act control (Shram and colleagues 2022, Clinical and Translational Science).
- "Difelikefalin works by sedating you so you don't notice the itch." — The anti-pruritic effect is mediated by peripheral KOR activation on sensory neurons and immune cells, modulating itch signaling at the source. Dizziness and somnolence are reported adverse events but are not the mechanism of action (Viscusi and colleagues 2021, Clinical and Translational Science).
- "All CKD patients with pruritus can use difelikefalin." — Korsuva is FDA-approved only for adults undergoing hemodialysis. Non-dialysis CKD patients and peritoneal dialysis patients fall outside the approved indication; oral difelikefalin for non-dialysis use is investigational only.
- "Difelikefalin will replace opioids for general pain management." — Approved use is limited to CKD-associated pruritus in hemodialysis patients. Earlier development in postoperative pain did not result in an approved analgesic indication.
Open questions
- Non-dialysis CKD pruritus: Oral difelikefalin is under investigation for non-dialysis CKD patients, but no formulation is approved for this population; access remains an unmet clinical need.
- Peritoneal dialysis: The KALM trials enrolled hemodialysis patients only; efficacy and appropriate use in peritoneal dialysis patients are not established.
- Other pruritic conditions: Phase 2 data in notalgia paresthetica have been published (Kim and colleagues 2023, NEJM); investigation in atopic dermatitis and other pruritic conditions is ongoing; none are approved.
- Long-term safety beyond two years: Open-label extension data extends to approximately 52 weeks to 2 years; cumulative cardiovascular, metabolic, or other safety effects at longer durations require continued pharmacovigilance.
- Mortality benefit: Observational data associate CKD pruritus with worse mortality outcomes (Saucereau and colleagues 2026, Renal Failure); whether effective pruritus treatment confers a survival benefit has not been tested in adequately powered mortality trials.
- Mechanism of dizziness and somnolence: These clinical signals persist despite peripheral receptor restriction; the underlying mechanism is not definitively established.
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Is this anti-itch drug's recorded target wrong, since difelikefalin is a known kappa opioid receptor agonist rather than a delta receptor binder?
Correcting the target record would help researchers reliably predict which patients and diseases could benefit and avoid testing the drug against the wrong pathway.
Could this kappa-targeting anti-itch drug also help patients with liver disease or pre-dialysis kidney disease, where similar peripheral itch pathways are involved?
If it works in these larger groups, it could offer relief to many more patients who currently have no approved itch treatment, improving sleep and quality of life.
Could the mirror-image amino acid structure of this drug change how it activates its receptor, on top of the established reason that it cannot reach the brain?
If a signaling difference were confirmed, it could add to why difelikefalin avoids the tolerance and dysphoria of older kappa drugs and inform safer peptide design.
▸full evidence table2 metrics
| metric | value | tool |
|---|---|---|
| ipTM | 0.9582021832466125 | boltz-2 |
| ranking score | 0.8207847476005554 | boltz-2 |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | colabfold_local |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-22 |
▸citationbibtex
@peptide{pep10805,
sequence = {FFFLK},
target = {oprd1},
author = {peptidemodel},
year = {2026},
status = {bioassayed}
}