Lack of Antiparkinsonian Effects of Systemic Injections of the Specific T-Type Calcium Channel Blocker ML218 in MPTP-Treated Monkeys

Abstract

Dopaminergic medications ameliorate many of the motor impairments of Parkinson's disease (PD). However, parkinsonism is often only partially reversed by these drugs, and they can have significant side effects. Therefore, a need remains for novel treatments of parkinsonism. Studies in rodents and preliminary clinical evidence have shown that T-type calcium channel (TTCC) antagonists have antiparkinsonian effects. However, most of the available studies utilized nonselective agents. We now evaluated whether systemic injections of the specific TTCC blocker ML218 have antiparkinsonian effects in MPTP-treated parkinsonian Rhesus monkeys. The animals were treated chronically with MPTP until they reached stable parkinsonism. In pharmacokinetic studies, we found that ML218 reaches a peak CSF concentration 1-2 h after s.c. administration. In electrocardiographic studies, we found no effects of ML218 on cardiac rhythmicity. As expected, systemic injections of the dopamine precursor L-DOPA dose-dependently increased the movements in our parkinsonian animals. We then tested the behavioral effects of systemic injections of ML218 (1, 10, or 30 mg/kg) or its vehicle, but did not detect specific antiparkinsonian effects. ML218 (3 or 10 mg/kg) was also not synergistic with L-DOPA. Using recordings of electrocorticogram signals (in one animal), we found that ML218 increased sleep. We conclude that ML218 does not have antiparkinsonian effects in MPTP-treated parkinsonian monkeys, due at least in part, to the agent's sedative effects.

Keywords: ML218; MPTP; Nonhuman primates; T-type calcium channel; behavior; parkinsonism; pharmacokinetic.

Figure 1. Pharmacokinetics of ML218 after systemic administration

Plasma and CSF time-concentration profiles of ML218 in Rhesus monkeys following a single s.c. dose (10 mg/kg) of ML218). Data are means + SEM

Figure 2. Electrocardiographic effects of ML218

Shown are measurements of RR, PR and RT intervals (means + SEM) in 3 animals (vehicle, 10 mg/kg, 30 mg/kg). Asterisks indicate significant differences between the data obtained with vehicle injections and those obtained with ML218.

Figure 3. Time course of behavioral experiments

The arrows indicate the times when L-DOPA (i.m.) or ML218 (s.c.) were systemically administered. A, B and C indicate sessions in which L-DOPA, ML218 or a combination of both drugs were administered.

Figure 4. Behavioral responses of parkinsonian animals to the administration of L-DOPA, ML218, or a combination of ML218 and L-DOPA

Each column represents the average (+ SEM) of data from 6 monkeys. For each experiment we calculated the median of all observation periods (5 consecutive periods of 15 min each). The baseline activity on the day of testing was subtracted from the post-injection values. Shown are number of movements (A) and activity counts (B). Statistical difference from respective vehicle: ** p<0.01; *** p<0.001 (Mann-Whitney test, followed by Bonferroni corrections).

Figure 5. Time course of behavioral changes after s.c. of L-DOPA, alone or in combination with ML218

The data shown are mean values (± SEM) of the number of movements (A) and activity counts (B) obtained for each observation period starting 15 min after L-DOPA injection (values are corrected to the pre-injection baseline and normalized to the number of minutes per observation period). ML218 (or the vehicle Tween 80) were administered 60 min before L-DOPA administration. The two experimental conditions were compared at each time point (Mann-Whitney test), no significant differences were found.

Figure 6. Results of ECoG recordings

A, example of 10 second epoch of M1 ECoG recording representing wakefulness and sleep. The scale bars in the top trace also apply for the bottom trace. Positive changes in voltage are upward. B, proportion of sleep epoch among all epochs (‘Sleep ratio’) obtained in the 2 hour period following the injection of Tween (white) and ML218 (grey). C, number of spontaneous movements in 5 minute epochs (out of 15 minute time blocks) obtained during 2 hours following the injections. D, spectral power of the ECoG signals, integrated across the 5 frequency bands (delta, theta, alpha beta, gamma). Values are means + SEM. The differences between ML218 and vehicle injection sessions were assessed with a Mann-Whitney test. *, p<0.05; **, p<0.01.