The objective of the study was to determine the effect of repeated administration on the pharmacokinetics and metabolism of diltiazem (DTZ) using an in vivo rat model. Male SD rats (n = 6-10 per group) weighing 350-450 g were used. Each rat received either a single 20 mg/kg dose of DTZ by subcutaneous (s.c.) injection or 5 mg/kg s.c. twice daily for five doses. Plasma concentrations of DTZ and its major metabolites were determined by HPLC for up to 8 h. Compared with the single dose, repeated administration resulted in higher dose normalized plasma concentrations of DTZ (AUC 26.4+/-14.2 vs 13.9+/-11.5 microg-h/ml), longer apparent half-life (t(1/2) = 12.5+/-14.6 vs 3.7+/-1.4 h) and lower systemic clearance (CL = 1.1+/-1.0 vs 2.9+/-2.7 l/h/kg). Higher dose normalized plasma concentrations, longer t(max), but shorter apparent t(1/2) of the major metabolites were observed following the repeated administration. The results also suggest that possible binding of DTZ may occur at the site of injection when administered subcutaneously in the higher dose.

The spinal cord contains neural circuits that can produce the rhythm and pattern of locomotor activity. It has previously been postulated that a population of glutamatergic neurons, termed Hb9 interneurons, contributes to locomotor rhythmogenesis. These neurons were identified by their expression of the homeobox gene, Hb9, which is also expressed in motor neurons. We developed a mouse line in which Cre recombinase activity is inducible in neurons expressing Hb9. We then used this line to eliminate vesicular glutamate transporter 2 from Hb9 interneurons, and found that there were no deficits in treadmill locomotion. We conclude that glutamatergic neurotransmission by Hb9 interneurons is not required for locomotor behaviour. The role of these neurons in neural circuits remains elusive.

In order to identify a suitable rodent model for preclinical study of calcium antagonists, pharmacokinetics and metabolism of diltiazem (DTZ) were compared in normotensive SD and hypertensive SHR rat models following multiple doses (5 mg/kg twice daily for 5 doses). Plasma concentrations of DTZ and its major metabolites appeared to be higher in the SHR than the SD rats, although the differences did not reach statistical significance (p > 0.05). The preliminary results suggest metabolism profile of DTZ in the SHR may be closer to humans than the SD rats and may be more preferred in pre-clinical drug development studies for DTZ.

AIMS To compare the pharmacokinetics and hemodynamic effect of diltiazem (DTZ) following single and repeated subcutaneous injection using an in vivo rat model. METHODS Male SD rats (n = 5 – 6 per group) weighing between 350 – 450g were used. Each rat received either a single 20 mg/kg dose of DTZ (Biovail Corp, Mississauga, Ont. Canada) by s.c. injection or 5 mg/kg s.c. bid for 5 doses. Plasma concentrations of DTZ and its major metabolites were determined by a previously published HPLC for up to 6h. In addition, hemodynamic measurements were recorded continuously for each animal during the experiment. RESULTS DTZ decreased SBP from 138 +/− 9 to 124 +/− 8 mmHg (10%) following the single dose and from 126 +/− 17 to 104 +/− 10 mm Hg (17%) following the 5th and last dose. The effects on DBP were also greater following the multiple doses (33% vs 26%), and the effects were also more sustainable after the repeated dosing. Plasma concentrations of DTZ were considerably higher after the single 20 mg/kg dose (AUC 8185 vs 3505 ng‐h/mL), although the half‐lives were longer at steady‐state (6 vs 2h). There were no apparent differences in the metabolite profiles. CONCLUSION Hemodynamic effects of DTZ were both qualitatively and quantitatively different between the single dose and steady‐state dosing. The reason is not completely clear although it may be related to greater tissue distribution of DTZ following repeated administration. (Supported in part by a grant‐in‐aid from CIHR/NSHRF/PEF Regional Partnership Program.) Clinical Pharmacology & Therapeutics (2005) 79, P53–P53; doi:

Although the differences were mainly quantitative and very small, the study has shown for the first time that the metabolism profiles of DTZ in SHR and WKY rats were closer to humans than SDR, and they may be more preferable rat models to study pharmacokinetic and metabolism studies of DTZ or similar agents.