What is the half life of moxidectin in humans
Executive summary
Clinical pharmacology studies and the FDA label report a long terminal elimination half‑life for oral moxidectin in humans: roughly 18–24 days in infected cohorts, with mean terminal values cited at 23.3 days (559 hours) after a single 8 mg dose; other clinical PK reports give ranges from about 17.7 up to 43 days depending on study and method [1] [2] [3] [4] [5].
1. What studies measured moxidectin’s half‑life in people — and what they found
Phase I/PK and infection‑cohort studies measured plasma concentrations over months and report a long terminal elimination half‑life for moxidectin in humans. Controlled pharmacokinetic work in people with Onchocerca volvulus found peak concentrations at ~3–4 hours and a prolonged elimination phase with half‑lives of about 18–24 days (mean values 17.7–23.3 days in that study) [1] [5]. Earlier and other trial data summarized in reviews and PK papers report mean elimination half‑lives broadly in the 20–35 day window, with some sources and reviews quoting wider ranges up to 43 days depending on assay and cohort [4] [3].
2. What the drug label and authoritative references state
The FDA prescribing information for moxidectin tablets gives an explicit mean terminal half‑life of 23.3 days (559 hours) following a single 8 mg oral dose in patients with onchocerciasis; the label also reports a very large apparent volume of distribution (about 2421 ± 1658 L) [2]. Commercial prescribing summaries repeat the 23.3‑day figure [6] [7].
3. Why reported half‑lives vary between studies
Reported half‑life varies because of study populations (healthy volunteers versus infected patients), single‑dose levels, sampling duration, and the PK model used to estimate terminal slope. Some studies note a pronounced early distribution phase (rapid fall over ~24 hours) followed by a long terminal elimination phase, producing mean terminal T1/2 estimates that span roughly 18–43 days across reports [1] [5] [3]. Different assays and the presence of low but measurable concentrations months later also widen the apparent range [1] [4].
4. How moxidectin’s half‑life compares with ivermectin and why that matters
Multiple sources stress that moxidectin’s terminal half‑life far exceeds ivermectin’s brief elimination (~1 day for ivermectin), a difference attributed to moxidectin’s greater lipophilicity, larger volume of distribution, and slower clearance; those properties underpin expectations for longer therapeutic exposure and potential single‑dose efficacy against parasites with multi‑week life cycles [3] [1] [8].
5. Clinical implications: dosing, efficacy and safety considerations
A long terminal half‑life supports the rationale for single‑dose regimens in onchocerciasis and may improve duration of suppression of microfilariae compared with ivermectin, as clinical trials and post‑marketing assessments have evaluated [5] [4]. However, long persistence also raises safety and special‑population questions (eg, pregnancy, lactation) noted in labels and reviews; available sources flag limited human lactation data and animal reproductive findings, and the label cautions about reproductive risks [2] [9].
6. Open questions and limitations in current reporting
Available sources do not provide a single, universal half‑life applicable to every patient or dosing scenario; instead they provide study‑specific ranges and mean estimates [1] [4] [2]. Data on tissue concentrations in humans (skin levels relevant to scabies) are limited despite animal evidence of retention, and sources note that skin PK and optimal dosing for indications beyond onchocerciasis require further study [8] [10]. Also, detailed information on plasma protein binding is inconsistent across summaries (some report unknown; others report very high binding) and should be consulted in the original label and PK papers for specifics [2] [7].
7. Bottom line for clinicians and patients
If you need a single, evidence‑based figure, the FDA label and multiple PK studies converge on a terminal half‑life near 3 weeks — mean ≈ 23.3 days after an 8 mg dose — with published clinical ranges commonly cited from roughly 18 up to 35 days and occasional broader estimates to ~43 days depending on the source and method [2] [1] [4] [3]. For decisions about dosing intervals, pregnancy, lactation, drug interactions or off‑label uses, consult the full FDA prescribing information and the primary PK literature referenced above [2] [1] [4].
Sources cited in text: FDA label and prescribing summaries [2] [6] [7], primary PK trials and PLOS/PMC articles [1] [5], clinical pharmacology reviews and trial summaries [3] [4] [8] [10].