Calcium channel antagonists: Part VI: Clinical pharmacokinetics of first and second-generation agents

Use of Cheminformatics to Determine Potential Drug Interactions between Popular Barbadian Botanical Medicines and Antihypertensive Drugs

Barbados has a rich traditional use of medicinal plants, especially among the older population who may have a chronic noncommunicable disease. This study aims to identify possible drug-herb interactions between popular herbal remedies used to manage elevated blood pressure and conventional antihypertensive drugs. In this study, in silico molecular docking experiments with AutoDock Vina (Scripps Research Institute, La Jolla, CA), a part of Yasara Structure software, version 20.12.24, were used to screen 30 potential phytochemicals for drug interactions from 11 popular plants in Barbados that are used for high blood pressure and could influence the pharmacology of the most prescribed antihypertensive drugs in Barbados. Thiazide and thiazide-like diuretics, calcium channel blockers (CCBs), angiotensin-converting enzyme inhibitors (ACE-I), and angiotensin receptor blockers (ARBs) are the most prescribed antihypertensive drugs. Twenty-seven phytochemicals show dissociation constants (K d) < 10 μM with pharmacological drug targets. Catharanthus roseus (L.) G. Don, Phyllanthus niruri L., Petroselinum crispum (Mill.) Fuss, and Lantana involucrata L. contain various compounds that show high binding affinities in all experiments. Possible interactions could affect renal excretion (thiazide-like diuretics), CYP metabolism (CCBs), absorption (ACE-I), hepatic CYP, and phase II metabolism (ARB). Oleanolic acid shows high binding affinities to almost all protein targets. This study also reveals potential candidates for the drug targets: T-type Cav3.3 (psychiatric diseases), PEPT1/2 (influencing bioavailability), and BK channel (epilepsy). Twenty-seven of 30 phytochemicals from C. roseus (L.) G. Don (Madagascar periwinkle), P. niruri L. (Seed under leaf), P. crispum Mill. Fuss (Parsley), and L. involucrata L. (Rock sage) have potential binding affinities with pharmacological targets of frequently prescribed antihypertensive drugs in Barbados and are likely to cause drug interactions. Compounds that are similar to naringin (e.g., astragalin, rutin, and quercitrin) and compounds that bind to OATP1, PEPT1/2, and enzymes involved in the metabolism of CCBs may be clinically relevant for further research. There should be greater awareness of potential drug-herb interactions, and further in vitro and in vivo studies are needed to unravel the exact effects on the pharmacology.

Calcium Channel Blocker Toxicity: A Practical Approach

Calcium channel blockers (CCBs) are widely prescribed medications for various clinical indications in adults and children. They are available in both immediate and long-acting formulations and are generally classified into dihydropyridines and nondihydropyridines, with nondihydropyridines having more cardioselectivity. CCB toxicity is common given the widespread use which leads to serious adverse clinical outcomes, especially in children. Severe CCB toxicities may present with life-threatening bradycardia, hypotension, hyperglycemia, and renal insufficiency. Dihydropyridine toxicity, however, may present with reflex tachycardia instead of bradycardia. Initial patient evaluation and assessment are crucial to identify the severity of CCB toxicity and design the best management strategy. There are different strategies to overcome CCB toxicity that requires precise dosing and close monitoring in various patient populations. These strategies may include large volumes of IV fluids, calcium salts, high insulin euglycemia therapy (HIET), and vasopressors. We hereby summarize the evidence behind the management of CCB toxicity and present a practical guide for clinicians to overcome this common drug toxicity.

1,4-Dihydropyridine derivatives as calcium channel modulators: the role of 3-methoxy-flavone moiety

It was earlier recognized that calcium antagonists, and in particular 1,4-dihydropyridines, exhibited distinct cardiovascular profiles. In addition two different splice variants of the L-type calcium channel were found in vascular and cardiac tissues. In this study, novel substituted 1,4-dihydropyridines with a 3-methoxy-flavone moiety were synthesized and structural modifications of the substituents in the dihydropyridine ring of nifedipine were carried out in order to find tissue specific compounds. The negative inotropic, chronotropic and vasorelaxant effects were investigated on guinea-pig left, right atria and aortic strips, respectively. The introduction of an heteroaromatic ring in 4-position of the 1,4-dihydropyridine nucleus led to compounds selective for cardiac tissues. Moreover, different residues in the 1,4-dihydropyridine ring could modulate the chronotropic versus inotropic activity.

Clinical pharmacokinetics of vasodilators. Part I

Understanding the mechanism of action and the pharmacokinetic properties of vasodilatory drugs facilitates optimal use in clinical practice. It should be kept in mind that a drug belongs to a class but is a distinct entity, sometimes derived from a prototype to achieve a specific effect. The most common pharmacokinetic drug improvement is the development of a drug with a half-life sufficiently long to allow an adequate once-daily dosage. Developing a controlled release preparation can increase the apparent half-life of a drug. Altering the molecular structure may also increase the half-life of a prototype drug. Another desirable improvement is increasing the specificity of a drug, which may result in fewer adverse effects, or more efficacy at the target site. This is especially important for vasodilatory drugs which may be administered over decades for the treatment of hypertension, which usually does not interfere with subjective well-being. Compliance is greatly increased with once-daily dosing. Vasodilatory agents cause relaxation by either a decrease in cytoplasmic calcium, an increase in nitric oxide (NO) or by inhibiting myosin light chain kinase. They are divided into 9 classes: calcium antagonists, potassium channel openers, ACE inhibitors, angiotensin-II receptor antagonists, alpha-adrenergic and imidazole receptor antagonists, beta 1-adrenergic agonist, phosphodiesterase inhibitors, eicosanoids and NO donors. Despite chemical differences, the pharmacokinetic properties of calcium antagonists are similar. Absorption from the gastrointestinal tract is high, with all substances undergoing considerable first-pass metabolism by the liver, resulting in low bioavailability and pronounced individual variation in pharmacokinetics. Renal impairment has little effect on pharmacokinetics since renal elimination of these agents is minimal. Except for the newer drugs of the dihydropyridine type, amlodipine, felodipine, isradipine, nilvadipine, nisoldipine and nitrendipine, the half-life of calcium antagonists is short. Maintaining an effective drug concentration for the remainder of these agents requires multiple daily dosing, in some cases even with controlled release formulations. However, a coat-core preparation of nifedipine has been developed to allow once-daily administration. Adverse effects are directly correlated to the potency of the individual calcium antagonists. Treatment with the potassium channel opener minoxidil is reserved for patients with moderately severe to severe hypertension which is refractory to other treatment. Diazoxide and hydralazine are chiefly used to treat severe hypertensive emergencies, primary pulmonary and malignant hypertension and in severe preeclampsia. ACE inhibitors prevent conversion of angiotensin-I to angiotensin-II and are most effective when renin production is increased. Since ACE is identical to kininase-II, which inactivates the potent endogenous vasodilator bradykinin, ACE inhibition causes a reduction in bradykinin degradation. ACE inhibitors exert cardioprotective and cardioreparative effects by preventing and reversing cardiac fibrosis and ventricular hypertrophy in animal models. The predominant elimination pathway of most ACE inhibitors is via renal excretion. Therefore, renal impairment is associated with reduced elimination and a dosage reduction of 25 to 50% is recommended in patients with moderate to severe renal impairment. Separating angiotensin-II inhibition from bradykinin potentiation has been the goal in developing angiotensin-II receptor antagonists. The incidence of adverse effects of such an agent, losartan, is comparable to that encountered with placebo treatment, and the troublesome cough associated with ACE inhibitors is absent.

Calcium antagonists in the elderly

The calcium antagonists are associated with a number of advantages over other antihypertensive agents, such as a lack of metabolic, vascular and respiratory adverse events, yet are effective in reducing blood pressure. The currently available calcium antagonists are widely used, and new members, particularly of the dihydropyridine group, continue to emerge. These agents may well be considered for use in the management of hypertension and angina in elderly people. They undergo significant first-pass metabolism and tend to have high values of hepatic clearance with minimal amounts of unchanged drug in the urine. Plasma concentrations tend to be higher in elderly people and for that reason it may be prudent to initiate therapy with lower dosages. With this caveat, adverse effect profiles seem to be qualitatively and quantitatively similar in younger and older people. At equivalent plasma concentrations, the antihypertensive effect appears similar in young and elderly patients, and clinical studies point to comparable efficacy with other drug classes. Calcium antagonists do not have adverse renal, respiratory, cardiovascular, metabolic or peripheral vascular effects and therefore may be useful in patients with relevant concomitant disease.

Different negative inotropic activity of Ca2(+)-antagonists in human myocardial tissue

To evaluate the negative inotropic effect of various Ca2(+)-antagonists in human myocardium without additional influences of preload, afterload, or frequency, we examined their effects on isometric force of contraction in isolated human papillary muscle strips and in auricular trabeculae. The 1,4-dihydropyridines isradipine, nitrendipine, and nifedipine, the phenylalkylamine verapamil, and the benzothiazepine diltiazem exerted concentration-dependent negative inotropic effects. The potency of the investigated Ca2(+)-antagonists was identical in papillary muscle strips of patients with only moderate clinical signs of heart failure undergoing mitral valve replacement-operation (NYHA II-III) and in terminally failing (heart transplantation, NYHA IV) human hearts. The IC50 values were lower in auricular trabeculae than in papillary muscle strips. The difference was significant for nifedipine, nitrendipine, and verapamil. The restorative effects of external Ca2+ after pretreatment with Ca2(+)-antagonists were significantly less strong after pretreatment with 1,4-dihydropyridine than with non-dihydropyridines in papillary muscle strips. It is concluded that 1,4-dihydropyridines and verapamil and diltiazem did differently influence Ca2(+)-mediated increase in force of contraction. Moreover, a relation between the therapeutically active free plasma concentration in vivo and the negative inotropic potency in vitro can be found. This relation follows a rank order of potency for negative inotropism (isradipine less than or equal to nitrendipine less than diltiazem less than nifedipine less than verapamil) and might have clinical relevance in the treatment of patients with compromised cardiac function.