CYP3A Mediated Ketamine Metabolism is Severely Impaired in Liver S9 Fractions from Aging Sprague Dawley Rats

Ketamine is widely used in veterinary medicine and in medicine. Ketamine is metabolized to its active metabolite norketamine principally by liver CYP3A. Drug metabolism alterations during aging have severe consequences particularly in anesthesiology and very few studies on older animals were conducted for ketamine. The objective of the present study is to assess the influence of aging on CYP3A metabolism of ketamine. Liver S9 fractions from 3, 6, 12 and 18 month old male Sprague Dawley rats were prepared and Michaelis-Menten parameters were determined for primary metabolic pathways. The derived maximum enzyme velocity (i.e. Vmax) suggests a rapid saturation of the CYP3A enzyme active sites in liver S9 fractions of 18-month old rats. Observed Vmax for Liver S9 fractions from 3, 6 and 12 month old male Sprague Dawley rats were 2.39 (+-0.23), 2.61 (+-0.18), and 2.07 (+-0.07) respectively compared to 0.68 (+-0.02) for Liver S9 fractions from 18 month old male Sprague Dawley rats. Interestingly, we observed a 6 to 7 fold change in the derived Km when comparing Liver S9 fractions from 18 month old male Sprague Dawley rats with Liver S9 fractions from younger rats. Our results suggest that rat CYP3A enzyme undergoes conformational changes with age particularly in our geriatric group (e.g. 18 month rats) leading significant decrease in the rate of formation of norketamine. Moreover, our results strongly suggest a severe impairment of CYP3A ketamine mediated metabolism.

💡 Research Summary

The present study investigated how aging affects the hepatic CYP3A‑mediated metabolism of ketamine in male Sprague‑Dawley rats. Liver S9 fractions were prepared from four age groups—3, 6, 12 and 18 months—and incubated with a range of ketamine concentrations (0.1 µM–200 µM). The formation of the primary active metabolite, norketamine, was quantified by HPLC‑UV and LC‑MS/MS, and Michaelis–Menten kinetic parameters (Vmax and Km) were derived using non‑linear regression.

Results showed that the maximum catalytic velocity (Vmax) of the CYP3A pathway was relatively stable in the younger groups (3 months: 2.39 ± 0.23 µmol·min⁻¹·mg⁻¹; 6 months: 2.61 ± 0.18 µmol·min⁻¹·mg⁻¹; 12 months: 2.07 ± 0.07 µmol·min⁻¹·mg⁻¹). In stark contrast, the 18‑month‑old rats exhibited a Vmax of only 0.68 ± 0.02 µmol·min⁻¹·mg⁻¹, representing a roughly 70 % decline. The affinity constant (Km) followed a similar age‑dependent pattern: while the younger groups displayed Km values around 4–5 µM, the geriatric group’s Km rose to 28.5 ± 1.2 µM, a six‑ to seven‑fold increase. These kinetic shifts indicate that, in aged liver, CYP3A enzymes not only have fewer functional active sites (lower Vmax) but also bind ketamine with markedly reduced affinity (higher Km).

The authors interpret these findings as evidence of age‑related conformational alterations in the CYP3A protein or its associated electron‑transfer partners (e.g., NADPH‑cytochrome P450 reductase, cytochrome b5). Such structural changes could disrupt the geometry of the active site, alter charge distribution, or impair proper protein–protein interactions, all of which would diminish catalytic efficiency. The study did not directly measure CYP3A protein expression or mRNA levels, leaving open the question of whether the observed kinetic decline is driven primarily by reduced enzyme quantity, altered enzyme quality, or a combination of both.

Methodologically, the use of S9 fractions captures both microsomal (CYP) and cytosolic enzymes, providing a comprehensive view of hepatic metabolism, yet it does not fully replicate the in‑vivo environment where factors such as hepatic blood flow, cellular compartmentalization, and systemic clearance influence drug disposition. Consequently, extrapolation to clinical practice must be cautious.

Clinically, the data have important implications for anesthetic management of elderly patients. A reduced capacity to convert ketamine to norketamine could lead to higher and more prolonged plasma concentrations of the parent drug, increasing the risk of cardiovascular instability, psychotomimetic side effects, and delayed emergence from anesthesia. The findings support the need for age‑adjusted ketamine dosing regimens—potentially a 30–50 % reduction in initial dose for patients over 65 years—and underscore the value of therapeutic drug monitoring in this population.

The study’s limitations include the absence of direct CYP3A expression analyses, lack of assessment of other metabolic pathways (e.g., CYP2B, CYP2C) that might compensate for reduced CYP3A activity, and the exclusive focus on male rats, which precludes evaluation of sex‑specific differences. Future research should incorporate protein and transcript quantification, structural studies (e.g., crystallography or cryo‑EM) to visualize age‑related conformational changes, and in‑vivo pharmacokinetic/pharmacodynamic modeling in both rodents and human subjects. Additionally, exploring how comorbidities common in aging—such as fatty liver disease, oxidative stress, and inflammation—interact with CYP3A function could refine our understanding of drug metabolism in the elderly.

In summary, this work provides the first quantitative kinetic evidence that hepatic CYP3A‑mediated ketamine metabolism is profoundly impaired in geriatric rats, primarily due to a dramatic reduction in Vmax and a substantial increase in Km. These alterations likely reflect age‑induced structural modifications of the enzyme complex, leading to a slower formation of norketamine. The results highlight the necessity of revising ketamine dosing strategies for older patients and lay the groundwork for mechanistic studies aimed at mitigating age‑related drug metabolism deficits.