Low central nervous system (CNS) penetration by darifenacin, a muscarinic M3 selective receptor antagonist, in rats

Damayanthi Devineni, Andrej Skerjanec & Thasia G Woodworth (Henry Danahay). Novartis Pharmaceuticals, East Hanover, New Jersey, USA.

Background: Darifenacin is a muscarinic M3 selective receptor antagonist developed for the treatment of overactive bladder (OAB). The potential risk of CNS side effects is likely to be influenced by drug concentrations in the brain. The propensity of darifenacin to enter the CNS has been investigated in two studies that monitored distribution of radio-labelled darifenacin in rats. Methods:* Study 1 was designed to qualitatively examine relative distribution in different tissues. Four male Lister hooded rats (217–240 g) received single intravenous injections of approximately 4 mg/kg [14C]darifenacin, and were killed 5 minutes or 1, 24 or 72 hours post-injection. Autoradiography was subsequently performed on frozen whole-rat sections. Study 2 expanded on the findings of Study 1, to specifically elucidate CNS penetration by darifenacin following oral administration (the method intended for clinical use), to male Sprague-Dawley rats (207–243 g; approximately 10 mg/kg [14C]darifenacin). Blood samples collected 1 or 4 hours post dose (5 mL; n=4 per timepoint) were centrifuged to obtain plasma, which was pooled before undergoing equilibrium dialysis. Cerebrospinal fluid (CSF) samples (50 µL) taken at the same timepoints were also pooled. Radioactive disintegrations in pooled plasma dialysate and CSF samples were evaluated by scintillation counting. HPLC profiling was used to elucidate relative concentrations of darifenacin and its metabolites.

In Study 1, autoradiography revealed low levels of radioactivity in rat brain relative to numerous other tissues including kidney, liver, lung, pancreas and spleen (Table 1), at each timepoint. In Study 2, radioactivity was detected in CSF at approximately one-tenth the concentration present in plasma dialysate (Table 2).

Table 1. Radioactivity levels (µg equivalent/g) in selected tissues; n=1 rat per timepoint. †

Values shown are average densities generated from multiple areas on autoradiograph sampled for each tissue; †study designed to permit qualitative assessment of relative distribution only.

Table 2. Radioactivity levels (ng equivalent/mL) in plasma dialysate and CSF; n=8 samples, pooled for analysis.

The majority of radioactivity in both plasma dialysate and CSF 1–4 hours post dose was detected as a metabolite of darifenacin (UK-73,689; 48% and 63% of total radioactivity in plasma dialysate and CSF, respectively), with minor fractions accounted for by two other metabolites (UK-346,036 and UK-148,993) and unchanged darifenacin. Of these metabolites, only UK-148,993 showed muscarinic binding activity in vitro, with 3–10 fold lower potency than darifenacin (EPAR). The contribution of UK-148,993 to biological activity has been found to be negligible (Kerbusch et al., 2003).

Darifenacin and its metabolites reached detectable levels in the CNS, but concentrations were markedly lower in the brain than in other major tissues. The low degree of partition into the CNS suggests that CNS adverse events may be minimised, consistent with the low incidence of such effects observed in clinical trials of darifenacin (Chapple et al., 2005).

Chapple, C., et al. (2005) BJU Int, 95, 993–1001.
Emselex European Public Assessment Report (EPAR), Scientific Discussion Kerbusch, T., et al. (2003) Br J Clin Pharmacol, 57, 170–180.

Studies were conducted in accordance with animal Ethics Committee approval.