Application of Physiologically Based Pharmacokinetic Modeling to Optimize Assessment of Age-Related Fluoxetine Accumulation in the Elderly.

Age-related decline in clearance, combined with fluoxetine metabolism and its conversion to norfluoxetine under the influence of CYP2D6 phenotype, leads to accumulation of the active moiety during prolonged therapy.

This work combines a physiologically based pharmacokinetic (PBPK) model of fluoxetine and its active metabolite norfluoxetine, implemented in the Simcyp Simulator and verified against published pharmacokinetic studies. Simulations of repeated 20 mg once-daily dosing were run in younger adults (18-65 years) and elderly individuals (65-98 years), then compared with therapeutic drug monitoring data from 47 Korean patients aged 18 to 88 years treated for at least five weeks. Simulations showed delayed steady state and reduced clearance in the elderly, with roughly two-fold higher fluoxetine exposure after prolonged dosing and 1.5-fold higher total active moiety exposure than in younger adults. Clinical observations pointed the same way, with significantly higher dose-normalized trough concentrations in elderly patients. CYP2D6 phenotype affected parent-drug exposure, but total active moiety exposure varied only modestly across phenotypes, leading the authors to consider phenotyping largely unnecessary when titrating fluoxetine in the elderly.

Synthesis written by Geno'X. For the full original abstract, please refer to the source publication.

This is first of all a simulation study: the clinical check rests on 47 patients monitored in routine care, with no detailed phenotype breakdown and no prospective comparison, which makes the conclusion that CYP2D6 phenotyping is unnecessary weaker than it sounds. The robust clinical message is age-related accumulation, which supports cautious titration and extended monitoring in older patients. Dropping phenotyping would require measured exposures by phenotype in an adequate sample, with efficacy and tolerability endpoints.

Analysis by Dr Thibaut Benquey