PII-105 - INVESTIGATION OF THE BIOTRANSFORMATION OF TRIHEXYPHENIDYL AND APPLICATION TO DYSTONIC CEREBRAL PALSY.

R. Gelineau-Morel^1,2,3, P. Toren¹, R. Pearce¹, J. Leeder¹; ¹Children's Mercy Kansas City, Kansas City, MO, USA, ²University of Missouri- Kansas City, Kansas City, MO, USA, ³University of Kansas, Kansas City, KS, USA.

Background: Trihexyphenidyl (THP) is an anticholinergic drug used to treat dystonia in cerebral palsy. Dystonia pathophysiology involves overexcitation of striatal cholinergic interneurons and rodent models demonstrate normalization of striatal activity after exposure to THP. Yet patients treated with THP demonstrate inconsistent efficacy and adverse effects, potentially due to variable systemic exposure. This project investigated THP biotransformation to inform individualized dosing and equalize systemic exposure.

Methods: THP and D11-THP were incubated with recombinant CYP2D6, CYP2C9, and CYP3A4 enzymes, and human liver microsomes. Analysis of metabolite formation in vitro and in vivo in 5 urine samples obtained from patients taking THP was conducted by liquid chromatography and tandem mass spectrometry.

Results: Incubation of THP with recombinant enzymes produced two hydroxylated metabolites: one associated with CYP2D6 (M1) and one with CYP3A4 (M2). Incubation of THP with human liver microsomes revealed predominant M1 formation at lower THP concentrations (0.2 µg/ml), which transitioned to predominant M2 at higher (supra-physiologic) concentrations (6 µg/ml). Patient urine samples revealed both M1 and M2, with predominance of M2. Incubation of D11-THP confirmed hydroxylation of the cyclohexane ring by both CYP2D6 and CYP3A4.

Conclusion: CYP2D6 and CYP3A4 hydroxylate the cyclohexane ring of THP and produce distinct metabolites that are present in the urine of patients taking THP. Future genotype-stratified pharmacokinetic analysis of THP will examine concentration-time profiles for individual patients. This will inform individualized dosing recommendations to equalize systemic exposure, improve treatment efficacy, and reduce adverse effects.

1. Eskow Jaunarajs KL, Scarduzio M, Ehrlich ME, Mcmahon LL, Standaert DG. Diverse Mechanisms Lead to Common Dysfunction of Striatal Cholinergic Interneurons in Distinct Genetic Mouse Models of Dystonia. The Journal of Neuroscience. 2019-09-04 2019;39(36):7195-7205. doi:10.1523/jneurosci.0407-19.2019
2. Downs AM, Donsante Y, Jinnah HA, Hess EJ. Blockade of M4 muscarinic receptors on striatal cholinergic interneurons normalizes striatal dopamine release in a mouse model of TOR1A dystonia. Neurobiol Dis. Jun 15 2022;168:105699. doi:10.1016/j.nbd.2022.105699
3. Sanger TD, Bastian A, Brunstrom J, et al. Prospective Open-Label Clinical Trial of Trihexyphenidyl in Children With Secondary Dystonia due to Cerebral Palsy. Journal of Child Neurology. 2007-05-01 2007;22(5):530-537. doi:10.1177/0883073807302601