Cyclobenzaprine
Cyclobenzaprine. Centrally-acting muscle relaxant — TCA-class structure; 5-HT2 antagonism + anticholinergic effect at brainstem reticular… Oral half-life 32 h.
Centrally-acting muscle relaxant — TCA-class structure; 5-HT2 antagonism + anticholinergic effect at brainstem reticular formation reduces tonic muscle activity. Sedating; used for acute musculoskeletal pain. Anticholinergic burden = Beers-list problematic in elderly.
PK: THE HALF-LIFE WAS VERBATIM FROM THE WRONG FORMULATION. The cited "mean t(1/2) was 33.4 hours for CER 15 mg and 32.0 hours for CER 30 mg" is EXTENDED-RELEASE cyclobenzaprine, filed against a record whose doses are immediate-release, and it also picks one of two arms. It becomes 31.9 h, verbatim from an immediate-release crossover sampled to 240 h. F 0.55 IS THE UPPER ENDPOINT of the label 33-55% range and becomes the only absolute measurement anyone has published: "the bioavailability of the 10 mg oral tablet, 10 mg i.m. and 20 mg i.m. injection was 0.33, 0.76, and 0.56, respectively, when compared to the 10 mg i.v. injection". A ROUTE MISMATCH WAS ONE KEYSTROKE AWAY in that sentence - 0.56 is the 20 mg INTRAMUSCULAR value. TWO OF MY PRIORS WERE WRONG: the half-life is not about 18 h, a figure I could not source anywhere, since two primaries on two formulations agree near 32 h; and a scalar F IS defensible after all, because 0.33 is a single intravenous-referenced absolute measurement and the 33-55% range is an artefact of secondary sources, so this record error was taking the wrong end of it. ka 0.4 is removed and is not even a back-derivation - it implies a 7.71 h peak, matching neither the extended-release 6.0 h nor the immediate-release 4.5 h. V 400 L has no quantitative source at all.
Pharmacokinetics
- Oral — t½ 31.9 h · F 33% · Vd 400 L
Sources: PMID:19243711, PMID:24151591, PMID:7082776