Eflornithine (DFMO)

Eflornithine (alpha-difluoromethylornithine, DFMO) is an enzyme-activated, irreversible suicide inhibitor of ornithine decarboxylase (ODC1, EC 4.1.1.17), the first and rate-limiting enzyme in the mammalian polyamine biosynthetic pathway. Synthesized in the late 1970s at the Merrell Dow Research Institute as a candidate antineoplastic agent, eflornithine was found to be ineffective as monotherapy against established human cancers but demonstrated potent antiprotozoal activity against Trypanosoma brucei gambiense in murine models. The compound was approved by the United States Food and Drug Administration in 1990 for the treatment of second-stage West African (gambiense) human African trypanosomiasis, marketed as Ornidyl. In 2000, a topical formulation (Vaniqa, 13.9% cream) received FDA approval for the reduction of unwanted facial hair in women, exploiting the compound's suppression of polyamine-dependent follicular cell division. The most recent regulatory milestone was the December 2023 FDA approval of an oral formulation (Iwilfin) to reduce the risk of relapse in adult and pediatric patients with high-risk neuroblastoma who have demonstrated at least a partial response to prior multiagent, multimodality therapy including anti-GD2 immunotherapy.

The molecular mechanism centers on covalent modification of ODC1. Eflornithine is decarboxylated by ODC1 with the assistance of the pyridoxal 5'-phosphate (PLP) cofactor, generating a reactive conjugated imine intermediate that alkylates the active-site cysteine-360 residue and irreversibly inactivates the enzyme. The functional consequence is depletion of intracellular putrescine and downstream spermidine and spermine pools, disrupting polyamine-dependent processes including DNA replication, mRNA translation, chromatin remodeling, and the LIN28/let-7 regulatory axis that is dysregulated in MYCN-amplified neuroblastoma. The compound exhibits a dissociation constant (KD) of approximately 1.3 micromolar and an inactivation rate constant (Kinact) of approximately 0.15 per minute against mammalian ODC1.

Pharmacokinetics are characterized by moderate oral bioavailability (54 to 58 percent), negligible plasma protein binding, absence of hepatic metabolism, renal excretion of approximately 86 percent of an administered dose as unchanged drug, and an elimination half-life of approximately 3 to 3.5 hours. The compound crosses the blood-brain barrier, achieving cerebrospinal fluid concentrations sufficient for trypanocidal activity in second-stage sleeping sickness. A landmark Phase III trial (Meyskens et al. 2008) demonstrated that oral DFMO at 500 mg daily combined with sulindac at 150 mg daily produced a 70 percent reduction in metachronous colorectal adenoma recurrence, a 92 percent reduction in advanced adenomas, and a 95 percent reduction in multiple adenomas compared to placebo over 36 months. A Phase III trial in familial adenomatous polyposis (Burke et al. 2020, New England Journal of Medicine) yielded mixed primary results but showed a 70 percent risk reduction in the subgroup of patients with intact colons. Preclinical data in TH-MYCN transgenic mice and neuroblastoma xenograft models support the clinical neuroblastoma application, demonstrating polyamine depletion, G1 cell cycle arrest, and tumor growth suppression. Adverse events at intravenous trypanosomiasis doses (400 mg/kg) include dose-dependent hematologic toxicity and reversible ototoxicity; at the lower oral chemoprevention and maintenance doses (500 to 750 mg daily), the compound is well tolerated with mild auditory changes at high frequencies as the principal dose-limiting signal.

This monograph reviews the chemistry, synthesis, and structural characterization of eflornithine; the irreversible enzyme-inhibitor mechanism in molecular detail; the comprehensive pharmacokinetic record; the preclinical pharmacology across parasitology and oncology models; the clinical evidence base spanning trypanosomiasis, hirsutism, neuroblastoma, and chemoprevention indications; sourcing and quality considerations; reconstitution and handling; stack-interaction implications; adverse-event profile; and a structured comparative assessment of five polyamine-pathway modulators against eflornithine on five competency standards.