PharmaceuticalAllosteric haemoglobin effectorRadiosensitiserOxygen-delivery modifier

Efaproxiral (RSR13)

Also known as RSR13 · Efaproxyn

Written by enwyReviewed 5 Aug 20262 sources0 comments

Efaproxiral (RSR13) is a synthetic allosteric modifier of haemoglobin that reduces oxygen affinity, increasing oxygen release to tissues. Trialled as a radiosensitiser for brain metastases, it failed to gain broad approval. It is explicitly named on the WADA Prohibited List as an oxygen-delivery modifying agent.

01 Overview

Efaproxiral binds haemoglobin and right-shifts the oxyhaemoglobin dissociation curve, so tissues extract more oxygen. Its lead clinical use was as a radiation sensitiser: better-oxygenated tumours respond more to radiotherapy. The pivotal REDOX/ENRICH-type programme for brain metastases did not deliver a clear enough benefit for general approval.\n\nBecause it improves tissue oxygen delivery, WADA lists efaproxiral by name among prohibited substances. Human exposure is limited to trials, so its safety in healthy athletes is not established; reported effects include transient hypoxaemia and headache.

02 Mechanism

Binds allosterically to haemoglobin, lowering its oxygen affinity and right-shifting the dissociation curve, so oxygen is released more readily to peripheral and tumour tissue.

03 Dosing

Tier	Dose	Route	Notes
Radiosensitiser (trials)	75–100 mg/kg	IV	Given before radiotherapy in trials; no approved athletic use

04 Effects

Effect	Magnitude	Evidence
Increased tissue oxygen deliveryTrials confirmed a measurable right-shift in the oxygen dissociation curve, increasing tissue oxygen availability.	Right-shifted p50	Clinical
Tumour radiosensitisationBetter-oxygenated tumours respond more to radiotherapy; this was its lead clinical indication, though benefit was insufficient for broad approval.	Modest, indication-specific	Clinical

05 Side effects

Effect	Severity	Frequency	Evidence	Countermeasures
Hypoxaemia / desaturationReduced haemoglobin oxygen affinity can lower arterial oxygen saturation, requiring supplemental oxygen in trial settings.	Moderate	Common in trials	Clinical
Headache and hypertensionHeadache and transient blood pressure changes reported in trials.	Mild	Common	Clinical

06 Legal status

United States

Not approved (investigational)

WADA-prohibited by name as an oxygen-delivery modifier · as of Jan 2026

07 References

Efaproxiral (RSR13) as a radiation sensitizer for brain metastasesJournal of Clinical Oncology, 2006

WADA Prohibited List — S2 (efaproxiral named)World Anti-Doping Agency

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.