The findings from this investigation must be considered to be preliminary in view of the limited number of subjects studied. In this context, verapamil as a single-agent mood stabilizer (as administered in Phase 2 of our study) showed minimal efficacy for treating manic patients who previously failed to respond to an initial three-week trial of lithium (30% response rate). There was no significant difference between verapamil treatment and continued administration of lithium during the three weeks of Phase 2, although the outcome with continued-lithium in Phase 2 was actually slightly better (50% response rate) than that with verapamil (30% response rate).
In contrast, patients who were treated with combined verapamil-lithium in Phase 3 showed substantial improvement that was statistically significant, compared to baseline, by the second week of treatment. This improvement was found regardless of whether the previous treatment used in Phase 2 had been verapamil or continued-lithium. It thus appears that combining verapamil with lithium may produce a clinically useful additive or synergistic effect. If these findings are confirmed by larger studies, adjunctive treatment with verapamil may become a novel strategy for effectively treating lithium-nonresponsive manic patients.
It should be noted that additive/synergistic actions such as those found in our study can have negative as well as positive consequences. Combined treatment with verapamil and lithium has been associated with a variety of adverse events, including exacerbation of lithium side effects (29
), choreoathetosis (30
), bradycardia with possible myocardial infarction (31
), and neurotoxicity with ataxia (32
). Thus, caution and appropriate monitoring are essential when using this drug combination.
It is important to note that the majority of patients in our study received concurrent treatment with antipsychotic medication. This likely reflects the severity of the manic episodes our subjects experienced, but also affects the interpretation of our findings. Since the antipsychotic medication was held constant in Phases 2 and 3 of the study, this should not have been a confounding variable in our results. Nevertheless, it cannot be determined from our findings whether concomitant use of antipsychotic medication may be essential for effectiveness of the combination treatment strategy reported here.
Historically, the rationale for using CCBs to treat mania rests on observations that baseline and/or stimulated intracellular calcium are elevated in bipolar patients, as measured in platelets (9
), lymphocytes (35
), and beta lymphoblast cell lines (37
). The inhibitory effect of verapamil on calcium uptake via L-type calcium channels is well known, and could potentially offset such increases. Lithium’s long-known actions on enzymatic and signal transduction components of the inositol phospholipid signaling system (39
) can likewise reduce mobilization of intracellular free calcium. For example, chronic exposure of beta lymphoblast cell lines to lithium was found to elevate resting calcium levels while attenuating stimulus-induced calcium mobilization (45
). In the hypothetical case where lithium fails to act sufficiently on calcium mobilization, a CCB could perhaps augment the effect on intracellular free calcium needed for therapeutic effectiveness.
However, Pazzaglia et al. (46
) suggested that verapamil may not substitute adequately for other CCBs. These authors reported that when verapamil was blindly substituted for nimodipine, two rapid cycling bipolar patients failed to maintain improvement, but responded again when nimodipine was reintroduced or replaced by isradipine. Thus, verapamil may differ from other CCBs in terms of its clinical actions, and it may be useful to consider other therapeutic mechanisms for this drug, especially with respect to combination treatment with lithium. In this regard, we note that the effects of the inositol phospholipid signaling system on intracellular calcium are paralleled by even more direct changes in the production of the intracellular second messenger DAG, which serves as an allosteric activator of PKC (47
As noted in a previous section, verapamil and lithium, as well as the antimanic agent valproate, all have actions on PKC. Current data suggest that long-term lithium exposure is accompanied by a down-regulation of specific PKC isozymes (48
). Studies in rodents have demonstrated that chronic (but not acute) lithium produces an isozyme-selective reduction in PKC α and ε
in the frontal cortex and hippocampus in the absence of significant alterations of the β, γ
, or ζ
). Concomitant studies in immortalized hippocampal cells in culture show a similar reduction in the expression of PKC α and ε
after chronic lithium exposure (48
). Furthermore, chronic lithium has been demonstrated to dramatically reduce the hippocampal levels of a major PKC substrate, myristoylated alanine rich C kinase substrate (MARCKS), a protein that has been implicated in regulating long-term neuroplastic events (51
It is noteworthy that the structurally dissimilar antimanic agent valproate produces very similar effects to those of lithium on PKC α and ε
isozymes and MARCKS protein (17
). Interestingly, lithium and valproate appear to bring about their effects on the PKC signaling pathway by different mechanisms (48
). These data further support a role for PKC modulation in the treatment of mania. Given the known inhibitory properties of verapamil on PKC, it seems appropriate to consider the possibility that the observations reported here, as well as other reports of antimanic actions of this drug in the literature, may in fact be related to PKC inhibition rather than calcium channel blockade, and that the actions, clinical specificity, and use in combination treatments of other CCBs lacking effects on PKC may differ from those of verapamil. Unfortunately, very little data exist in terms of the potential isozyme specificity of verapamil’s actions. Thus, although a synergistic action for verapamil and lithium could potentially arise from differential effects on PKC isozymes, this must remain speculative at present. Further research on this issue seems warranted.
In addition to the findings on drugs effects discussed above, extensive preclinical and clinical research supports the notion that PKC is an appropriate target for antimanic treatments. This family of closely related enzyme subspecies plays a major role in the regulation of neuronal excitability, neurotransmitter release, and long-term alterations in gene expression and neuronal plasticity (57
). Animal studies support a role for PKC in regulating the release of dopamine (56
), a neurotransmitter widely believed to be implicated in manic episodes. Manic-like behaviors such as increased hedonistic drive, increased tendency to use drugs, hyperactivity, and risk-taking behavior are attenuated by PKC inhibitors (14
). Finally, excessive activation of PKC dramatically impaired the cognitive functions of the prefrontal cortex, whereas inhibition of PKC protected cognitive function (13
A limited number of human studies also support the idea that PKC abnormalities occur in bipolar disorder. Although perhaps an over-simplification, particulate (membrane) PKC can be viewed as the more active form of this enzyme, and thus measurement of the subcellular partitioning of PKC can be utilized to estimate the degree of activation. With this approach, Friedman et al. (64
) reported that measures of membrane-bound PKC activity were elevated in platelets from manic patients, and that lithium treatment over a two-week period reduced cytosolic and membrane-associated PKC activities, and attenuated PKC activation. Wang and Friedman (65
) measured PKC isozyme levels, activity, and translocation in postmortem brain tissue, and reported increased PKC activity and translocation in bipolar disorder brains compared to controls, with elevated levels of selected PKC isozymes in the cortex. The same group subsequently reported that postmortem brains from bipolar disorder subjects showed increased association with receptor for activated C kinase-1 (RACK1) (66
). Since PKC is anchored to the membrane via RACK1, these results suggest that increased association of RACK1 with PKC isozymes may be responsible for the previously observed increases in membrane PKC and in its activation.
Finally, because activation of PKC by DAG is regulated by diacylglycerol kinases (67
), the findings from two recent independent genome-wide association studies of bipolar disorder are of interest. Baum et al. found diacylglycerol kinase eta (DGKH) to be the most highly significantly associated (p ~ 10−8
) bipolar susceptibility gene (22
). The second study, from the Wellcome Trust in the UK (68
), reported on different single-nucleotide polymorphisms (SNPs) than the Baum et al. study, but several SNPs in DGKH showed association with bipolar disorder at the 10−3
level (data available at http://www.wtccc.org.uk/info/summary_stats.shtml
). Two of these SNPs were in the same region as those found to be highly significant in the Baum et al. study. Since DAG is the major activator of PKC, these results suggest that PKC signaling abnormalities may well be etiologically
involved in bipolar disorder, and lend support to the potential utility of pharmacologic strategies targeting PKC in the treatment of mania.
A weakness in the design of our study was that the combined treatment was only administered to subjects after six weeks of protocol participation. Thus, a possibility exists that the findings may have been confounded by spontaneous recovery from the manic episode in some individuals. This issue could be addressed in future studies by including a verapamil-lithium combined treatment arm as one of the initial treatments. However, it should be noted that the response rates as shown in for Phase 3 single-agent treatments cannot serve as an index of spontaneous recovery, because subjects assigned to single-agent treatment in Phase 3 had by definition already responded to that treatment in the previous phase.
Another potential weakness of our design is that subjects were treated with verapamil for only three weeks in Phase 2. If the optimal response to verapamil occurs only after a delay of more than three weeks, as is observed with several other psychotropic medications, then this could be a confounding factor in the findings regarding combination treatment. Although two of the three patients treated with verapamil alone in Phase 3 showed a positive outcome, these patients were all verapamil responders during Phase 2. We cannot rule out the possibility that treatment of Phase 2 verapamil nonresponders for an additional three weeks with verapamil alone could have produced a positive outcome as a result of longer duration of treatment. On the other hand, our linear mixed model analysis of the Phase 3 data showed that the decrease in mania scores in Phase 3 was significant regardless of whether the previous Phase 2 treatment had been verapamil or lithium. Thus, groups who received verapamil for a total of either three or six weeks both showed significant improvement with Phase 3 combination treatment. Nevertheless, it is possible that due to relatively lower lipid solubility of verapamil, longer exposure time may be needed to build up adequate brain concentrations of the drug.
In conclusion, this study provides new preliminary evidence that augmentation of lithium (± antipsychotic) treatment with verapamil can improve therapeutic outcome in manic patients who do not respond to an initial trial of lithium (± antipsychotic). As in the treatment of other disorders such as hypertension and epilepsy, our results suggest that rational combination therapy may have considerable utility in the treatment of this difficult disorder. Additional research could also determine whether this combination treatment will allow lower doses of individual agents to be used, potentially reducing the burden of adverse effects. Future investigations should attempt to replicate our findings in a larger sample and study combined verapamil-lithium as an initial and potentially more robust antimanic treatment. Given the evidence for involvement of PKC in the actions of antimanic drugs, and the ability of verapamil to attenuate PKC activity, the prima facie assumption that this drug’s antimanic action is exclusively based on calcium channel blockade should be reconsidered. Further study of PKC and its isozymes in relation to therapeutic outcome with verapamil and more established antimanic agents seems warranted.