Bioavailability study of a concentrated suspension of phenytoin in healthy volunteers

The bioequivalence of a new concentrated suspension of phenytoin was compared with the standard reference solution recommended by the United Kingdom Committee on Review of Medicines in a group of 18 healthy male volunteers. Doses of 5ml (94.6mg) of the new formulation and 50ml (92mg) of the reference solution were administered in a randomised crossover design and blood samples were withdrawn over the next 72 hours. Serum concentrations were analysed by high performance liquid chromatography. There were no significant differences in the areas under the plasma concentration-time curves of the two formulations although the time to peak was shorter and peak concentrations were higher with the reference solution. As the total amount of phenytoin absorbed from each preparation was not significantly different, both formulations have the same relative bioavailability.

This new concentrated phenytoin suspension may have clinical advantages over tablets, capsules and the commercially available low strength suspension, in patients who have difficulty in swallowing but require normal adult doses of phenytoin

Lercanidipine: a novel lipophilic dihydropyridine calcium antagonist with long duration of action and high vascular selectivity

Lercanidipine is a new 1,4-dihydropyridine derivative with potent, long-lasting and vascular-selective calcium entry blocking activity. Animal models of hypertension have shown lercanidipine to be potent, with a slow rate of onset and long lasting action and to have minimal or no effects on cardiac contractility. There was no evidence of tolerance after repeated oral treatment, and no effects were found on the autonomic nervous, central nervous, gastrointestinal or respiratory systems at antihypertensive doses. In man, lercanidipine is well absorbed after oral administration, with peak plasma levels occurring approximately 1.5 - 3 h after dosing. The drug is subject to extensive hepatic first pass metabolism with an elimination half-life of 2 - 5 h. With a more sensitive method, a mean terminal elimination half-life of 8 - 10 h was defined. Despite this short plasma half-life the drug has a long duration of action, most likely due to the high lipophilicity of lercanidipine and its partitioning in to the lipid bilayer of cell membranes, followed by diffusion to the receptor binding site. The efficacy of lercanidipine has been established in extensive clinical trials with comparison to both placebo and standard well-established antihypertensive therapies. These trials confirmed the efficacy of lercanidipine and its long duration of action which renders it suitable for once daily administration. Tolerability was good in all studies: the adverse event profile was comparable to that of placebo at lower doses, with a low incidence of palpitations and ankle oedema. Lercanidipine is a recently introduced example of a lipophilic and vasoselective dihydropyridine calcium antagonist which is an effective antihypertensive drug with a slow onset and long duration of action; it is associated neither with reflex tachycardia nor cardio-depressant activity.

Clinical comparative trials of angiotensin II type 1 (AT1)-receptor blockers

Three characteristics of antihypertensive medication are pivotal in therapy for a patient with hypertension: potency and efficacy, duration of action, and the incidence of side-effects. The relatively new class of AT1-receptor blockers all display placebo-like tolerability and, as a class, their antihypertensive efficacy compares well with other antihypertensive classes. However, it is unclear whether clinically important differences in duration of action and antihypertensive efficacy exist within the AT1-receptor blocker class itself. The results of a number of head-to-head clinical comparisons between these agents suggest that candesartan cilexetil and irbesartan may be more effective than the prototype AT1-receptor blocker, losartan. In addition, studies using ambulatory blood pressure recording techniques have clarified the relative durations of antihypertensive action of the AT1-receptor blockers. In particular, studies mimicking the common event of a missed or delayed dose of antihypertensive medication, show that the antihypertensive effect of candesartan cilexetil extends well beyond the 24-h dosing interval, while the effect of losartan declines rapidly over this period. The available evidence therefore suggests that significant differences in efficacy and duration of action are apparent within the AT1-receptor blocker class, and that these differences may well translate into clinically relevant differences in cardiovascular outcome.

Is postural hypotension a real problem with antihypertensive medication?

Historically, postural hypotension has been cited as a consideration which might influence the selection of antihypertensive therapy. The common symptoms (dizziness, blackouts, syncope) give cause for concern but they are not attributed to every class of antihypertensive drug. For example, administration of a beta-blocker is not generally associated with symptomatic postural hypotension, whereas the alpha-blocker prazosin was particularly problematical, with a significant and well-recognized risk of first-dose postural effects. Titration from a low starting dose and careful selection/monitoring of patients have been successfully used to circumvent this problem. However, since there is a relatively high incidence of postural hypotensive symptoms in elderly patients in general, it may be a misconception to attribute such symptoms to any particular type of antihypertensive drug. Furthermore, with the newer alpha(1)-blockers, such as doxazosin, which have a more gradual onset of action, there is a markedly reduced tendency for postural hypotension to occur. Thus, it is perhaps time to reassess the real significance of iatrogenic postural hypotension in the selection of antihypertensive therapy.

The importance of sustained blood pressure control

The case for antihypertensive drug regimens that produce consistent 24-h blood pressure control has largely been founded upon a series of epidemiological observations that were either cross-sectional or alternatively relatively small-scale follow-up studies. More recent data have unequivocally demonstrated, in a prospective study in hypertensive subjects with left ventricular hypertrophy, that the reduction in left ventricular mass index during the course of one year's antihypertensive treatment, was predicted much more closely by treatment-induced changes in ambulatory blood pressure than by changes in clinic blood pressure. This provides definitive and confirmatory data to support the aim of achieving blood pressure control, which is based upon a smooth and consistent antihypertensive effect over a full 24-h dosage interval. Regimens which provide such control may also offer the advantage of a sustained duration of effect beyond 24 h. This characteristic is attractive because even the most compliant patient may inadvertently miss at least one dose of medication each week. Evidence from a number of studies which have sought to mimic this pattern of suboptimal compliance by deliberately inserting a placebo phase into a steady-state treatment regimen, has clearly demonstrated the benefits of antihypertensive drugs with intrinsically long duration of action. Furthermore, there is evidence to suggest that following cessation of therapy there is a biphasic reversion of blood pressure towards baseline levels with a maintenance of a residual effect which is more pronounced with a long-acting agent when compared to a shorter-acting drug from the same therapeutic class. There is increasing evidence, albeit not derived from prospective outcome studies, that indicates that the benefits of antihypertensive therapy are likely to be maximized by treatment regimens which result in sustained blood pressure control.

Intrinsically long-acting agents compared with long-acting chronoformulations

Intrinsically long-acting antihypertensive drugs may be characterized by long elimination half-lives with high trough: peak ratios for decreasing blood pressure. These agents are usually administered once daily in the morning, and at steady-state they provide 24 h blood pressure control, attenuate the early-morning surge in blood pressure, and maintain a normal circadian blood pressure pattern. In comparison, chronoformulations incorporate shorter-acting antihypertensive drugs into a delivery system that is delayed-onset or extended-release, or both. These agents, which are often designed to be given once daily at bedtime, deliver peak drug concentrations that coincide with the early-morning surge in blood pressure. Chronoformulations also provide 24 h blood pressure control. In highly compliant patients, both intrinsically long-acting drugs and chronoformulations are likely to provide comparable blood pressure control. However, in poorly compliant patients who miss doses of medication, intrinsically long-acting drugs are likely to be superior, because they sustain blood pressure control beyond the dosing interval.

Evaluation of an Antihypertensive Therapy Utilising Meta-analysis of a Clinical Trial Database

The emphasis on evidence-based medicine and the use of safe, cost-effective therapeutic strategies demands that management decisions be based upon evidence derived from clinical studies. Whilst prospective double blind clinical trials are regarded as the gold standard, there is no doubt that meta-analysis has proved valuable in defining the benefits of antihypertensive therapy. The antihypertensive efficacy of lacidipine has been assessed in a retrospective meta-analysis of a series of clinical studies in which the drug was compared with placebo and all the major classes of antihypertensive agent. All of the studies entered into the meta-analysis were parallel group double blind trials. Efficacy was based upon the reduction in both systolic and diastolic blood pressure and the effect on heart rate at trough immediately prior to dosing during maintenance therapy. In placebo controlled trials a clear dose response relationship was apparent with blood pressure reductions that were significantly greater than that observed with placebo at doses of 2 mg lacidipine and above. In active control trials, diastolic blood pressure reductions of 10-15 mmHg were observed at the end of the monotreatment phases (> 6-8 weeks) with a final reduction of 15-20 mmHg with the efficacy of lacidipine being equivalent to that of the comparator drug. Comparable results were also achieved for the response rates to therapy which varied between 70 and 85% at the end of the monotreatment phase and 82-98 when combination with other antihypertensive agents was permitted. There was no evidence of cardio-acceleration in any of the trials where significant blood pressure reduction was detected. This retrospective analysis in a large population base confirms the documented antihypertensive efficacy of lacidipine and demonstrates the suitability of the drug as a first line antihypertensive agent.

Trough: peak ratio and smoothness index for antihypertensive agents

There is a convincing volume of evidence to support the contention that optimal control of blood pressure should be based upon therapeutic strategies that consistently reduce blood pressure in a smooth and consistent fashion. Attention has, therefore, been focused on calculation of trough:peak ratios and, alternatively, the smoothing index as methodologies for defining the duration of action of an antihypertensive drug and for discriminating among alternative treatments. Acceptable accuracy and reproducibility for trough:peak ratio have been demonstrated in the controlled environment of a research unit. In contrast, trough:peak ratios from ambulatory blood pressure recordings exhibit wide inter-patient variability. With respect to clinical validity, unlike trough:peak ratio, the smoothing index has been shown to be correlated to the regression of left ventricular hypertrophy induced by treatment. Overall, neither index has been proven to offer definitive superiority and hence it is reasonable to suggest that the two are complementary.

Optimal dosing characteristics of the angiotensin II receptor antagonist telmisartan

An optimal antihypertensive drug produces superior blood pressure-lowering effects at established dosages, with an acceptably low incidence of side effects, and at a dosage interval that is convenient for patients (ideally, once daily). The angiotensin II receptor antagonist, telmisartan, meets these criteria. At doses of > or = 40 mg, this once-daily drug produces a statistically significant reduction in blood pressure. Ambulatory blood pressure monitoring (ABPM) and high trough-peak ratios attest to the smooth, consistent blood pressure-lowering effect of telmisartan at 40- and 80-mg dosages. Telmisartan also demonstrates a statistically superior antihypertensive effect toward the end of the dosing interval compared with amlodipine and losartan, and it has a side-effect profile comparable to that of placebo. In summary, the evidence suggests that telmisartan at dosages of 40 and 80 mg once daily satisfies the 3 criteria of an ideal antihypertensive agent, producing an effective and sustained response with placebo-like tolerability.

Mortality amongst patients of the Glasgow Blood Pressure Clinic was high in the 1970s and 80s but has fallen since, why?

Established in 1968 the Glasgow Blood Pressure Clinic has over 11,000 patients on its computer record. Up to 1980, mortality from all-causes and from cardiovascular causes was high: relative risks compared with two local control populations were greater than 2.0. Since 1980, all-cause mortality has decreased to 1.31 (859 deaths, CI 1.23-1.39). Lower mortality from cardiovascular causes, particularly coronary heart disease, contributes to the decrease. Reasons for the decrease are under investigation currently. Referral of patients with slightly lower blood pressure contributes, as may better blood pressure control with newer antihypertensive drugs. ACE inhibitors and calcium channel blockers were introduced in 1980 and during the 16-year period to 1995, all-cause mortality has decreased most in patients taking ACE inhibitor. A decrease also occurred in patients taking antihypertensive drugs other than ACE inhibitor.

Is cancer related to hypertension or to its treatment?

Three questions related to cancer and blood pressure are discussed. (i) Is cancer related in some way to hypertension, or to blood pressure? Several studies show a relation of blood pressure and cancer in populations. However, our own experience, based on a cohort of 15,411 subjects with BP measured in the 1970s and with 1,392 fatal cancers since, shows no relation of cancer risk and diastolic pressure. Nor were cancer numbers (n=72) observed in the 1,078 untreated hypertensives of the Glasgow Blood Pressure Clinic different from those expected (n=71.2) in a control population matched for age, sex and smoking habit. (ii) Do antihypertensive drugs promote cancer? Atenolol and calcium channel blockers have been suspected of this, but evidence of larger studies, including two of our own, is negative: relative risk for cancer in our patients taking CCB was 1.02 (CI 0.82-1.27). (iii) Do antihypertensive drugs protect against cancer? A study of ours based on the Glasgow Clinic raises this possibility: relative risk for incident cancer amongst 1,559 patients taking ACE inhibitor was 0.72 (CI 0.55-0.92).

ACE (I/D) genotype as a predictor of the magnitude and duration of the response to an ACE inhibitor drug (enalaprilat) in humans

BACKGROUND

We have investigated the possible effects of contrasting ACE (I/D) genotypes on the responses to the ACE inhibitor enalaprilat in normotensive men.

METHODS AND RESULTS

Subjects with DD (n=12) and II (n=11) ACE genotypes received an intravenous infusion of enalaprilat or placebo. Pressor responses to stepwise, incremental doses of angiotensin I were measured at 1 and 10 hours after dosing. The dose required to raise mean blood pressure by 20 mm Hg (PD20) was calculated individually, and the ratio of PD20 during enalaprilat to that during placebo (dose ratio, DR) was used for assessment of the extent of ACE inhibition. The pressor response was significantly attenuated at 1 hour after enalaprilat in both groups, but significant attenuation was evident at 10 hours after dose only in the II subjects. The DRs at both 1 hour (median, 5.43 versus 2.82, P=0.0035) and 10 hours (2.06 versus 0.84, P=0.0008) after enalaprilat were significantly higher in II subjects than in DD subjects.

CONCLUSIONS

The effect of enalaprilat was significantly greater and lasted longer in normotensive men homozygous for the II ACE genotype. By multivariate analysis, ACE (I/D) genotype and plasma angiotensin II levels were predictive of >50% of the variation in response to ACE inhibition.

Do inhibitors of angiotensin-I-converting enzyme protect against risk of cancer?

BACKGROUND

Previous studies have reported an increased risk of cancer with calcium-channel blockers in man. Other work in animals suggests that inhibitors of angiotensin-I-converting enzyme (ACE) protect against cancer. We aimed to assess the risk of cancer in hypertensive patients receiving ACE inhibitors or other antihypertensive drugs.

METHODS

Our retrospective cohort study was based on the records of 5207 patients who attended the Glasgow Blood Pressure Clinic between Jan 1, 1980, and Dec 31, 1995. The patients' records are linked with the Registrar General Scotland and the West of Scotland Cancer Registry.

FINDINGS

Compared with the West of Scotland controls, the relative risks of incident and fatal cancer among the 1559 patients receiving ACE inhibitors were 0.72 (95% CI 0.55-0.92) and 0.65 (0.44-0.93). Among the 3648 patients receiving antihypertensive drugs other than ACE inhibitors (calcium-channel blockers 1416, diuretics 2099, beta-blockers 2681), the corresponding relative risks were 110 (0.97-1.22) and 1.03 (0.87-1.20). The relative risk of cancer was lowest in women on ACE inhibitors: 0.63 (0.41-0.93) for incident cancer; 0.48 (0.23-0.88) for fatal cancer; and 0.37 (0.12-0.87) for female-specific cancers. The reduced relative risk of cancer in patients on ACE inhibitors was greatest with follow-up of longer than 3 years. Calcium-channel blockers, diuretics, and beta-blockers had no apparent effect on risk of cancer.

INTERPRETATION

Long-term use of ACE inhibitors may protect against cancer. The status of this finding is more that of hypothesis generation than of hypothesis testing; randomised controlled trials are needed.

How to evaluate the duration of blood pressure control: the trough:peak ratio and 24-hour monitoring

Recently published data provide confirming evidence to support existing epidemiologic data showing that treatment-induced changes in 24-h blood pressure are more closely correlated with treatment-induced changes in left ventricular mass index than corresponding changes in clinic blood pressure. This provides definitive support for the aim of achieving blood pressure control based on a smooth and consistent antihypertensive effect over a full 24-h period. With the plethora of agents now available for once-daily administration in the treatment of hypertension, it would be of value to have a validated index that defines duration of effect and that discriminates between alternative treatments and treatment regimens. The trough:peak ratio of blood pressure response has been proposed as such as index and has in part been validated in this role. Evidence suggests that many existing agents have suboptimal trough:peak ratios or, alternatively, that achieving a high ratio is dependent on utilizing inappropriately high doses of drug. Where a drug has a high trough:peak ratio that is associated with its intrinsic long duration of action and is independent of dose over the therapeutic range, the evidence indicates that the drug may offer the additional benefit of sustained blood pressure control after a missed dose. Finally, theoretic and experimental evidence supports the concept that rational drug combinations will produce enhanced trough:peak ratios compared to the individual drug components.

Evaluation of endpoints in hypertension: blood pressure

The benefits of antihypertensive therapy in reducing both cerebrovascular and cardiac events have been clearly demonstrated in the meta-analysis of randomised outcome trials. Whilst the use of diuretics and beta-blockers have tended to predominate in these trials, other agents were also included and thus it is reasonable to suggest that the benefit of treatment is not attributable to any particular class of agent but rather to a reduction in blood pressure per se. It may therefore, be reasonably argued that blood pressure itself is the only validated surrogate marker of cardiovascular outcome. In routine clinical practice evaluation has indicated that in treated hypertensives not only is blood pressure not lowered to normotensive levels but also that control of pressure was not consistent over a 24 hour period. Finally epidemiological evidence suggests that blood pressure control should be based upon treatment strategies that lower blood pressure to normotensive levels in a smooth and consistent fashion.

Role of trough to peak efficacy in the evaluation of antihypertensive therapy

BACKGROUND

The major outcome trials clearly demonstrate that there is benefit associated with treatment of hypertension, not only in a reduced incidence of stroke but also in a reduction in coronary heart disease. The latter reduction is, however, less than might be anticipated from epidemiological evidence. TWENTY-FOUR-HOUR BLOOD PRESSURE CONTROL: Although there is still no definitive evidence that 24-h blood pressure control will lead to improved outcomes compared with drugs that provide intermittent control, there is a large body of evidence showing that cardiovascular target organ damage is correlated with 24-h blood pressure measurements and supportive evidence showing that a fall in these measurements can predict a probable reduction in cardiovascular target organ damage. TROUGH:PEAK RATIO AS AN INDEX OF BLOOD PRESSURE CONTROL: The Food and Drug Administration guidelines on trough:peak ratio offer an index not only of the 'safety' of an antihypertensive agent but also of its duration of action over the recommended dosage interval. Ideally, an agent should have a trough:peak ratio that consistently exceeds 60% and does so throughout the recommended therapeutic dose range. When this aim is achieved, particularly with agents that have intrinsic long duration of action, the evidence suggests that blood pressure control is sustained well beyond the dosage interval, providing 'cover' for the poorly compliant patient.

CONCLUSION

Epidemiological evidence indicates that optimal antihypertensive therapy should be based upon achieving smooth and consistent blood pressure control over a full 24 h. This is most likely to be achieved by long-acting antihypertensives that are characterized by having a high trough:peak ratio.

Cancer risk of hypertensive patients taking calcium antagonists

OBJECTIVE

To measure rates of incident and fatal cancer in hypertensive patients taking calcium antagonists and to compare these with rates in three control groups.

DESIGN

A retrospective analysis of cancer in patients of the Glasgow Blood Pressure Clinic prescribed either a calcium antagonist or other antihypertensive drugs (non-calcium antagonist group). Record linkage of the clinic with the West of Scotland Cancer Registry and with the Registrar General, Scotland provided information on incidence of cancer and on deaths and their causes.

PATIENTS

2297 patients were prescribed calcium antagonist and 2910 were prescribed antihypertensive drugs other than calcium antagonist.

MAIN OUTCOME MEASURES

Relative risk of cancer, the ratio of observed to expected cancers in the calcium antagonist group, was estimated using expected values based on three control groups; namely the non-calcium antagonist group, a middle-aged population of Renfrew and Paisley and the West of Scotland population.

RESULTS

There were 134 incident cancers in the calcium antagonist group, representing relative risks of 1.02 [95% confidence interval (CI) 0.82-1.271 compared with the non-calcium antagonist group, 1.01 (95% CI 0.84-1.18) compared with Renfrew-Paisley controls and 1.02 (95% CI 0.85-1.19) compared with West of Scotland controls. Findings for cancer mortality were similarly negative. Risks were no higher for older patients.

CONCLUSIONS

Our study lends no support to the suggestion that calcium antagonists cause cancer.

Clinical relevance of optimal pharmacokinetics in the treatment of hypertension

BACKGROUND

Although it is well recognized that many antihypertensive drugs exhibit large interpatient variability in their disposition characteristics, the concept that this translates into a high variability in antihypertensive response, by some form of concentration-effect relationship, has largely been ignored.

PHARMACOKINETICS AS A DETERMINANT OF RESPONSE

Inferentially, there is a volume of evidence that the pharmacokinetic characteristics of an antihypertensive drug translate, in part, to the haemodynamic characteristics. Thus, agents with an intrinsically long elimination half-life and low variability in drug clearance tend to produce a sustained and consistent reduction in blood pressure. PHARMACOKINETIC-PHARMACODYNAMIC MODELLING: With a rigorous approach to study design, the pharmacokinetic and pharmacodynamic responses to many antihypertensive drugs can be integrated, and thus the concentration-effect relationship can be modelled. This approach has been used on a population basis to model the blood pressure, heart rate and P-Q interval responses to mibefradil. The results of this analysis have demonstrated clearly that for blood pressure and the heart rate, the baseline or pretreatment characteristics of the patients are important determinants of the response.

CONCLUSION

For many antihypertensive drugs, the circulating concentrations are an important, if not the most important, determinant of response. Characterizing concentration-effect relationships is an important route to optimizing antihypertensive drug therapy.

Clinical pharmacokinetics of nifedipine. Implications for the care of the elderly

Nifedipine, the prototype for the dihydropyridine class of calcium antagonists, has been available for 20 years and its efficacy as a vasodilator and an antihypertensive agent is well recognised. The development of the so-called nifedipine gastrointestinal therapeutic system (GITS), which allows once-daily administration, has modified and improved the overall therapeutic profile of nifedipine to such a significant extent that it might almost be considered a new drug entity. The nifedipine GITS is associated with distinct improvements in terms of patient compliance and convenience, and a reduced incidence of adverse effects. With regard to the care of the elderly, this 'new' drug offers the prospect of a well tolerated and effective treatment without major cost implications.

Importance of trough : peak ratios in treatment of blood pressure in post-myocardial infarction and heart failure settings

Hypertension increases the risk of reinfarction and sudden death in the post-myocardial infarction patient, and the same also applies to the co-existing left ventricular hypertrophy (LVH). Thus the therapeutic aim in such patients are: (1) appropriate blood pressure (BP) control; (2) regression of LVH; and (3) secondary prevention to reduce long-term mortality. All of these goals can potentially be met by use of ACE inhibitors. However, achieving quality BP control demands appropriate selection of doses and dosage regimens. Trough : peak ratios are useful in determining duration of action and may thus offer a rational approach to aid the selection drug doses and dosage regimens.

Concentration-effect relationships and implications for trough-to-peak ratio

The guidelines on trough-to-peak ratio identified an index of the duration of action of an antihypertensive drug (relative to its dosage interval) to prevent the use of inappropriately large doses of drug simply to extend the apparent duration of action. In some instances, however, trough-to-peak ratio may be dose-dependent and this analysis examines the contribution that the underlying concentration-antihypertensive effect relationship makes to the dose dependency of trough-to-peak ratio. Where this concentration-effect relationship is essentially linear the trough-to-peak ratio is almost invariably dose-independent. In contrast, where the relationship is identified as being of a sigmoid-Emax type the trough-to-peak ratio is likely to be dose-dependent. The nature of the concentration-effect relationship also influences the duration of action beyond the dosage interval whereby "linear" drugs are superior to "Emax" drugs by virtue of the greater persistence of the antihypertensive effect.

Calculation of trough-to-peak ratio in the research unit setting. Advantages and disadvantages

The trough-to-peak ratio for the response to an antihypertensive drug is a clinically meaningful parameter but only when the calculation has been derived from an appropriate and scientifically robust study. Since the methodological details have not been defined by any regulatory authority, several possible approaches have developed. The major apparent advantages of the intensive study of individual patients in the research unit setting are that the conditions of measurement can be standardized and an accurate account can be taken of the circadian variations in the responses to placebo and active drug treatment. The principal disadvantage is that it is an "artificial" environment that may, or may not, be directly relevant to routine clinical circumstances. Nevertheless, the values obtained with this approach to date are directly comparable to values obtained by the alternative approaches, such as ambulatory blood pressure measurements (provided that those are also well-conducted studies). Thus, using the trough-to-peak ratio not only appears valid but also permits the detailed study of individual patients and also lends itself to the incorporation of additional and confirmatory clinical pharmacological assessments.

Generic drugs. Therapeutic equivalence

For economic reasons, the use of generic substitution is increasingly being supported by health authorities. Potentially, this may be problematic for drugs with a narrow therapeutic window if quality control and/or bioequivalence is not optimal. Many developing countries do not have the resources or expertise to carry out appropriate quality control resulting in widespread distribution of substandard or even counterfeit drugs. Even in countries where procedures are well regulated, substandard drugs reach the market from time to time. Interchangeability of drugs is determined by bioequivalence studies comparing the serum concentration versus time curves for the products following single dose administration to fasting volunteers in a randomised crossover design. A number of reports, largely anecdotal, of treatment failure or increased adverse events after switching brands has cast some doubts upon whether bioequivalence testing is sufficient in all cases. These reports have covered cardiovascular, respiratory, hormonal, psychotropic, anticonvulsant, anti-infective and anti-inflammatory drugs. Equivalence is particularly difficult to obtain with many sustained-release formulations. The WHO has initiated programs to prevent the distribution of substandard preparations and has drafted guidelines for testing bioequivalence based on internationally accepted reference products. Until such time as means can be provided-first, to enforce internationally accepted production standards, and second, to permit uniform testing of therapeutic agents-the safest clinical choice, particularly in countries where registration requirements and quality control are minimal, must remain the branded product.

Dose-ranging study of the angiotensin II receptor antagonist irbesartan (SR 47436/BMS-186295) on blood pressure and neurohormonal effects in salt-deplete men

We characterised the blood pressure (BP) and hormonal responses to the oral angiotensin II (Ang II) receptor antagonist irbesartan (SR47436/BMS-186295) or placebo in normal men with an activated renin-angiotensin system (RAS) during salt depletion. We also evaluated safety and tolerability. Twelve healthy, normotensive male volunteers followed a standardised salt-depletion regimen for 3 days before each study day. Six different single oral doses of irbesartan (1, 5, 10, 25, 50, and 100 mg) were administered double-blind in a three-panel, dose escalation with placebo randomised in each panel. Supine and erect BP and heart rate (HR), serum and urinary electrolytes: plasma renin activity (PRA), and Ang II were measured at intervals. Urinary electrolytes were measured for the 24-h period before dosing (to confirm salt depletion) and for 24 h afterward. No drug-related side effects were noted. There was a dose-related decrease in supine and erect systolic and diastolic BP (SBP, DBP) with irbesartan from 10 mg and beyond, with no change in HR. Supine mean arterial pressure (MAP) decreased by 18.8 mm Hg. There was a dose-related reactive increase in PRA (to 35 ng/ml/h) and Ang II (to 450 pg/ml) with irbesartan. Irbesartan is an orally active AT1 receptor antagonist. In salt-deplete normal men, it has a dose-related haemodynamic, hormonal, and electrolyte profile characteristic of AT1 antagonists. The dose range studied did not show a plateau or maximum effect.

Implications of the links between hypertension and myocardial infarction for choice of drug therapy in patients with hypertension

Meta-analyses of major outcome trials have demonstrated that the benefits of antihypertensive treatment for reduction of the incidence of stroke are entirely consistent with the benefits predicted from epidemiologic data; however, there remains a shortfall in the expected reduction of the incidence of coronary heart disease. Several explanations have been proffered to account for this shortfall, including the potential deleterious metabolic effects of long-term antihypertensive treatment; this has led to the speculation that antihypertensive agents with beneficial ancillary properties might confer additional significant advantage. However, with the exception of the angiotensin-converting enzyme inhibitors, few of these agents have translated into clinical benefit for humans. In addition, sound reasons exist to justify a focus on maintaining and improving the "quality" of blood pressure control. Current evidence suggests that optimal benefit is likely to result from the use of pharmacologic strategies that lower blood pressure consistently over a 24-hour period while at the same time maintaining the "normal" circadian pattern of blood pressure. This result will only be achieved with drugs and drug regimens that genuinely offer long duration of action with the additional potential benefit of maintaining a significant blood pressure lowering effect beyond the end of the dosage interval. This factor is particularly important because many patients with hypertension demonstrate poor adherence to prescribed dosage regimens.

Therapeutic implications of drug "holidays'

While a patient may appear to be fully compliant with respect to quantity of medication ingested, the timing of doses can vary considerably. This may introduce protracted intervals between doses, particularly if dosing frequency is greater than once daily. This commonly presents in the form of drug holidays, where dosing is omitted for 1 or more days, followed by a resumption of full-strength dosing, possibly resulting in excessive drug effects when dosing suddenly resumes, rebound effects when dosing is suddenly stopped and a period without effective drug action. Poor compliance has not only been associated with poorer treatment outcome, but also with financial consequences. It has been suggested that hospitalization due to non-compliance accounts for 11.7% of all healthcare expenditure in the U.S.A. Differences between drugs, in terms of both their concentration-time profiles and their duration of action, will lead to some agents being more effective than others in the face of interruptions in dosage. A drug with an intrinsically long duration of action, such as the calcium antagonist amlodipine, will provide better therapeutic coverage than those with a shorter duration of action and therefore minimize effects generated by an intermittent pattern of dosing.

Haemodynamics, cardiac conduction and pharmacokinetics of mibefradil (Ro 40-5967), a novel calcium antagonist

OBJECTIVES

Mibefradil (Ro40-5967) is a chemically novel non-dihydropyridine calcium antagonist. In this phase II study we compared its acute and chronic effects on blood pressure, heart rate and atrioventricular conduction (electrocardiographic PQ interval) with those of verapamil and diltiazem.

PATIENTS AND METHODS

After a 4-week placebo run-in, 18 patients with mild to moderate essential hypertension were given single doses of mibefradil (150 mg), slow-release (SR) verapamil (240 mg), diltiazem (240 mg) and placebo at weekly intervals; pharmacokinetics and the effects on blood pressure, heart rate and PQ interval were studied on four 10-h study days. Seventeen of the same patients subsequently underwent 4 weeks of treatment with either mibefradil (100 mg daily; n = 10) or verapamil SR (240 mg daily; n = 7), and on the last day, they attended a further 10-h study day. Two studies were conducted: an acute, single-dose, double-blind, randomly allocated, placebo-controlled, crossover study and a chronic, open-label, randomly allocated, parallel-group study.

RESULTS

Mibefradil was well tolerated. In the acute study, the antihypertensive effect (difference from placebo) of mibefradil 150 mg was of slower onset than that of verapamil or diltiazem, but comparable blood pressure reductions had been achieved by 6 h. The mean +/- SD maximal PQ prolongation (difference from placebo) was 15.6 +/- 16.1 ms, compared with 44.0 +/- 22.6 ms for verapamil and 56.0 +/- 48.9 ms for diltiazem (P<0.05 mibefradil versus verapamil; P<0.01 mibefradil versus diltiazem). In the chronic study there were no significant differences during steady-state conditions between mibefradil at 100 mg and verapamil SR at 240 mg in their effects on blood pressure, PQ and heart rate. The mean +/- SD elimination half-life (t1/2) of mibefradil under steady-state conditions was 26.8 +/- 5.5 h (versus an apparent t1/2 of 16.9 +/- 11.1 h for verapamil SR, P<0.05).

CONCLUSIONS

Mibefradil is a well-tolerated and efficacious antihypertensive agent well suited to single daily dosing because of its intrinsic long plasma half-life. The effects on both blood pressure and PQ interval are of more gradual onset than those of unmodified verapamil and diltiazem after single doses.

Organ protection and optimal blood pressure control

The morbidity and mortality associated with hypertension demonstrates the need to direct antihypertensive therapy towards reducing target organ damage, particularly those that are well validated predictors of high risk. It is now well recognized that closer relationships exist between hypertensive organ disease and blood pressure from 24-h ambulatory blood monitoring as opposed to conventional clinic determinations. Furthermore, two further important features have been apparent in studies using 24-h blood pressure measures. First, patients who fail to exhibit a "normal" nighttime circadian fall in blood pressure show a higher incidence of cardiovascular and cerebrovascular complications. Second, there is convincing evidence to suggest that blood pressure variability over a 24-h period is an important independent determinant of target organ damage. This evidence suggests that optimal benefit will result from the use of drugs that effectively lower blood pressure over a full 24 h dosage interval, and that this smooth blood pressure effect should be superimposed upon the normal circadian blood pressure pattern and at the same time reduce blood pressure variability. It is likely that this will only be achieved by the use of genuine long-acting antihypertensive drugs with a high trough:peak ratio.

The Effects of Age on the Pharmacokinetics and Pharmacodynamics of Cardiovascular Drugs: Application of Concentration-Effect Modeling. 2. Acebutolol

The effects of age on the disposition and hemodynamic responses to the selective beta(1) adrenoceptor antagonist were examined in an integrated manner using a combined pharmacokinetic--pharmacodynamic (PKPD) modeling approach. Thirty subjects were studied to represent, as far as was possible an age continuum. Single doses of acebutolol (20 mg) and placebo were administered intravenously and blood was withdrawn and heart rate and blood pressure were measured at frequent intervals up to 10 h post dose. No significant effects of age on the disposition of acebutolol and its major acetylated metabolite were apparent. Acebutolol when compared with placebo produced significant changes in blood pressure and heart rate and application of PKPD modeling demonstrated a significant negative correlation between blood pressure responsiveness to acebutolol and age.

Effects of Age on the Pharmacokinetics and Pharmacodynamics of Cardiovascular Drugs: Application of Concentration-Effect Modeling. 3. Trimazosin

The effects of age on the disposition and hemodynamic responses to the selective post-synaptic alpha(1) adrenoceptor antagonist trimazosin were examined in an integrated manner using a combined pharmacokinetic--pharmacodynamic (PKPD) modeling approach. Thirty subjects were studied to represent, as far as was possible an age continuum. Single doses of trimazosin (100 mg) and placebo were administered intravenously and blood was withdrawn and heart rate and blood pressure were measured at frequent intervals up to 10 h post dose. Based on regression analysis there was a statistically significant decline in the clearance of trimazosin with increasing age. In addition, based on the ratio of the AUC values for the major metabolite and parent drug, there was evidence of a decline in the relative clearance of matabolite with increasing age. Trimazosin when compared with placebo produced significant changes in blood pressure and heart rate that were statistically greater in elderly subjects. PKPD modeling revealed that both trimazosin and its metabolite 1-hydroxy trimazosin contributed significantly to the hemodynamic profile of the drug but the blood pressure responsiveness to both parent drug and metabolite were unaffected by age. Thus the greater response in the elderly subjects could be attributed to the decline in drug clearance with age.

The Effects of Age on the Pharmacokinetics and Pharmacodynamics of Cardiovascular Drugs: Application of Concentration-Effect Modeling. 1. Tolmesoxide

The effects of age on the disposition and hemodynamic responses to the direct acting vasodilator tolmesoxide were examined in an integrated manner using a combined pharmacokinetic--pharmacodynamic (PKPD) modeling approach. Thirty subjects were studied to represent, as far as was possible, an age continuum. Single doses of tolmesoxide (100 mg) and placebo were administered intravenously and blood was withdrawn and heart rate and blood pressure were measured at frequent intervals up to 10 h post dose. No significant effects of age on the disposition of tolmesoxide and its sulfone metabolite were apparent. Tolmesoxide when compared with placebo produced significant changes in blood pressure and heart rate but PKPD modeling failed to demonstrate any significant effects of age on the blood pressure response to tolmesoxide.

Differential effects of a missed dose of trandolapril and enalapril on blood pressure control in hypertensive patients

A double blind randomised comparison of two angiotensin-converting enzyme (ACE) inhibitors was made in a study in which ambulatory blood pressure was monitored over a steady-state dosage interval and the subsequent 24-h period, the latter being designed to mimic a missed dose of drug. The blood pressure responses on active therapy were compared to an identical recording made at the end of a 3-week placebo run in period. Eighty-eight essential hypertensives were treated with a morning dose of either trandolapril 2 mg or enalapril 20 mg. Mean systolic (SBP) and diastolic blood pressure (DBP) were calculated on each of the following periods: daytime (8:31 a.m.-10:30 p.m.), nighttime (10:31 p.m.-6:30 a.m.), and early morning (6:31 a.m.-8:30 a.m.). Trough/peak was calculated for each group both on active treatment and after a missed dose. Twelve patients were excluded from analysis before opening the randomisation code because of inadequate ambulatory blood pressure monitoring (ABPM) recordings. Demographic data, placebo-period office blood pressure, and ABPM recordings were not significantly different between the two groups. Both trandolapril and enalapril effectively reduced blood pressure over the 24-h period. Twenty four-hour ambulatory SBP and DBP decreased from 148 +/- 14/92 +/- 10 mm Hg to 135 +/- 14/83 +/- 10 mm Hg in the trandolapril group (p < 0.001). The same parameters decreased to a quite similar extent after enalapril, from 143 +/- 13/91 +/- 5 mm Hg to 133 +/- 15/83 +/- 8 mm Hg (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of trough:peak ratio and the assessment of anti-hypertensive drug action

The conventional blood pressure (BP) measurement is essentially a 'snapshot' at a single time point in any given 24 h period. In the clinical management of the hypertensive patient, however, it is assumed that this snapshot is representative of the BP throughout 24 h and indicative of the consistency of the control produced by anti-hypertensive drug treatment. However, anti-hypertensive drugs vary in the duration and consistency of their anti-hypertensive effect and a single measurement of BP can only be reliably indicative if the drug effect is known to be consistently maintained throughout the 24 h. Following the deliberations of the Food and Drug Administration in the USA, the calculation of a trough:peak ratio has been proposed as an index of the consistency of the anti-hypertensive response and the suitability of an anti-hypertensive drug for its chosen dose and dose interval. In brief, to confirm that the magnitude of the BP is relatively consistent throughout 24 h it is recommended that the magnitude of the BP reduction at the end of the dose interval (at trough) should be at least 50% of the BP reduction measured at peak, namely a trough:peak of 50%. The trough:peak ratio provides a clinically relevant index for assessing whether an anti-hypertensive drug is likely to provide a consistent anti-hypertensive effect throughout the 24 h.

Comparison of the oral angiotensin II receptor antagonist UP 269-6 or enalapril 20 mg on blood pressure and neurohormonal effects in salt-deplete man

We compared the response of the oral angiotensin II (Ang II) receptor antagonist (ARA) UP 269-6 with an angiotensin converting enzyme inhibitor (ACEI) enalapril 20 mg or placebo, during salt depletion in normal men. We also evaluated safety and tolerability. Sixteen healthy, normotensive male volunteers followed a standardised salt-depletion regimen for 3 days before each study day. Seven different doses of UP 269-6 (5, 10, 20, 40, 80, 120 and 180 mg) were administered double blind in a four-panel dose escalation, with enalapril and placebo randomised within each panel. Supine and erect blood pressure (BP) and heart rate (HR); serum and urinary electrolytes; plasma active renin (PAR), aldosterone, and Ang II were measured at intervals. Urinary electrolytes and aldosterone were measured for the 24 h before dosing and for 24 h after dosing. Dizziness and light-headedness on standing were reported after UP 269-6 at higher doses. Enalapril caused one episode of symptomatic postural hypotension. No other drug-related adverse events (AE) were noted. There was a dose-related decrease in supine and erect systolic and diastolic BP (SBP, DBP) with UP 269-6 at > or = 40 mg, with no change in HR. Based on the maximal decrease in mean arterial pressure (MAP), UP 269-6 at 180 mg had an effect largely comparable to that of enalapril 20 mg. There was a dose-related increase in PAR with UP 269-6. Although this was greater with UP 269-6 180 mg than with enalapril, serum and 24-h urinary aldosterone suppression was greater with enalapril than with any dose of UP 269-6.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood pressure variability and its implications for antihypertensive therapy

Although it is clear that antihypertensive treatment is beneficial in reducing stroke morbidity and mortality, the results of the major outcome studies show less impact on coronary heart disease. Studies utilizing 24-h blood pressure (BP) monitoring show a positive association between target organ damage and the level of 24-h BP, and with variability in BP, which is an independent determinant of target organ damage. Current understanding of the pathogenesis and pathophysiology of coronary heart disease suggests that optimal antihypertensive treatment should ensure the following: effective 24-h BP control, smooth antihypertensive effect with reduced variability; attenuation of the early morning surge in BP; maintenance of the normal circadian pattern of BP; effective therapeutic coverage in the face of suboptimal compliance; and lack of reflex activation of the sympathetic nervous system. On the basis of our current understanding, this optimum is most likely to be achieved by the use of antihypertensive agents with a long duration of action.

New FDA guidelines on the treatment of hypertension: comparison of different therapeutic classes according to trough/peak blood pressure responses

The FDA guidelines, recommending a trough/peak blood pressure response ratio of at least 50%, were formulated with a view to providing a definitive index of duration of action of an antihypertensive drug. The aim was to prevent the use of drug regimens that utilised high doses of drug with the aim of maintaining a significant reduction in blood pressure at the end of the steady state dosage interval. The calculation of trough/peak ratios is subject to significant variability but much of this can be directly attributed to different methodological approaches. However, when conditions are standardised it has been shown that trough/peak ratios are reproducible in individual patients. Trough/peak ratios defined for different antihypertensive drugs often exhibit as many differences within a therapeutic class as between therapeutic classes. Thus there is no single therapeutic class of drugs that offers high trough/peak ratios compared to an alternative class. The possible exception to this are the diuretics which probably all have comparatively high trough/peak ratios although this has never formally been defined. With respect to the beta adrenoceptor antagonists there is discernible discrimination between the once a day agents with betaxolol, bisoprolol and acebutolol all having a longer duration of action as defined by a higher trough/peak response than atenolol. Calcium antagonists show considerable variability in trough/peak ratio between different drugs. In particular the first generation agents, verapamil, nifedipine and diltiazem all had relatively low values. Not all the second generation agents were superior to this and at present only amlodipine and lacidipine and some of the "reformulated" agents meet the recommendations of the Guidelines for once a day drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Methodological considerations in calculation of the trough: peak ratio

AIM

The Food and Drug Administration in the United States has published guidelines which indicate that a minimum trough: peak ratio of 50-66% is required for the efficacy of an antihypertensive drug to be considered satisfactory in relation to its proposed dosage interval. However, these guidelines do not give any definition of the most appropriate methodology, and published data contain widely disparate values which often reflect methodological inconsistencies. This article attempts to define the principal methodological requirements for the accurate and reproducible measurement of trough and peak antihypertensive effects and for calculation of the trough: peak ratio. PROBLEMS IN CALCULATING THE TROUGH:PEAK RATIO: It is essential to take account of the antihypertensive effect of placebo, otherwise the results may be spurious. Similarly, account must be taken of the closely related circadian variability in blood pressure, which is particularly likely to compromise the interpretation of the peak effect. While the incorporation of placebo in a randomly allocated, crossover design is ideal, there are practical (and ethical) difficulties with this approach.

CONCLUSIONS

Provided there is an adequate placebo run-in period (of not less than 4 weeks) and individual patients are studied under carefully standardized conditions, with multiple blood pressure recordings throughout the dose interval, it is possible by means of a sequential, placebo-active treatment design to calculate the trough: peak ratio with acceptable accuracy and reproducibility.

Antihypertensive treatment and trough: peak ratio: general considerations

AIM

The proposed United States Food and Drug Administration guidelines on the trough: peak ratio were established to define the duration of action of an antihypertensive drug and to prevent the 'apparent' duration of action being extended by inappropriately large doses of drugs. Some studies have indicated that the trough:peak blood pressure response to some antihypertensive drugs is dose-dependent. The aim of the present investigation was to determine the relationship between drug concentration and its effect on the trough:peak ratio, particularly when the antihypertensive effect extends beyond the dose interval.

LINEAR VERSUS SIGMOID RELATIONSHIP BETWEEN BLOOD PRESSURE FALL AND DRUG CONCENTRATION

Where the relationship between effect and concentration is linear the effect of the dose on the trough:peak ratio is minimal. In contrast, where the relationship is sigmoid (Hill equation), the value of the trough:peak blood pressure response is dose-dependent. Drugs with linear relationships between concentration and effect are more likely to sustain a useful antihypertensive response beyond the dose interval than sigmoid drugs. Published studies on the response to missed drug doses support this theoretical differentiation in drug type.

CONCLUSIONS

We conclude that trough:peak blood pressure responses provide a useful arithmetic index of the duration of action of an antihypertensive drug. The nature of the relationship between concentration and effect determines the influence of the dose on the trough:peak ratio and whether the duration of action is extended beyond the dose interval.

Amlodipine--does the effect-time profile directly reflect the concentration-time profile throughout a 24-hour period?

Two studies were undertaken to investigate correlations between the time profile of plasma drug concentration and the time profile of hypotensive activity for the dihydropyridine calcium antagonist, amlodipine. The first study compared the concentration- and effect-time profiles for amlodipine and felodipine ER (extended release), and the second study characterized the concentration-effect relationship in hypertensive patients. The inter- and intra-subject variabilities in the disposition characteristics of amlodipine were less than for felodipine and there was correspondingly less variability in plasma drug concentrations; trough-to-peak ratios were calculated as 67 +/- 8% for amlodipine and 36 +/- 13% for felodipine ER. The characteristics of the plasma-concentration profiles appeared to be reflected in the profiles of hypotensive response such that, although the peak effect with felodipine ER was greater, there was less variability with amlodipine and the trough effect with amlodipine was consistently superior. Examination of the relationship between the plasma concentrations of amlodipine and the antihypertensive effect revealed that, although there was a temporal discrepancy between the two profiles, the two could be correlated using a linear-effect model. Results of this analysis indicated that the kinetic-dynamic model was most appropriately fitted simultaneously to the acute and steady-state data. Thus, the low inter- and intra-subject variabilities in the disposition characteristics of amlodipine are translated into a consistent and smoothly sustained 24-h antihypertensive response.

Prediction and optimisation of the antihypertensive response to nifedipine

The predictability of the long term antihypertensive response to nifedipine in individual patients has been assessed by an analysis based upon the concentration-effect parameters defined following the first dose administration of 20 mg nifedipine (Retard). The predicted and measured reductions in blood pressure during steady state nifedipine treatment were compared for the trough and peak responses and there was reasonable agreement for the group of patients as a whole. However, when the measured and predicted blood pressure profiles were compared for each individual patient there was close agreement for the majority of patients but there were significant discrepancies in a few cases. Further analysis of the steady state concentrations in these cases revealed that there was no change in their responsiveness to nifedipine and that discrepancies were directly attributable to inappropriate compliance with the drug regimen. The analysis was further extended to simulate the blood pressure responses to alternative fixed dosage regimens. Assessment of these simulations suggests that blood pressure control with nifedipine Retard is significantly improved by three times daily drug administration.

Concentration-effect analysis of antihypertensive drug response. Focus on calcium antagonists

Although individualised antihypertensive therapy is widely recommended, prospective methods for optimising treatment are hampered by the paucity of basic information about dose-plasma concentration-response relationships for commonly used drugs. Concentration-effect analysis has been applied to a number of therapeutic areas. With antihypertensive drugs this approach has clearly identified direct relationships between pharmacokinetic and pharmacodynamic profiles within individual patients. Thus, either a linear or nonlinear model can be used to quantify the antihypertensive drug response in terms of parameters that incorporate pharmacokinetic and pharmacodynamic information. Furthermore, these models take account of placebo effects and time-dependent changes in blood pressure and drug concentrations during a dosage interval. Concentration-effect analysis has been used to characterise the responses to a range of calcium antagonist drugs. These studies have demonstrated that these analyses are useful for optimising dosage schedules, identifying determinants of blood pressure response, and predicting steady-state profiles of blood pressure (including peak/trough effects) after administration of a single ('test') dose. This mode of analysis warrants early inclusion in the clinical development of any new antihypertensive agent, so that the familiar difficulties in identifying the optimum dosage range are avoided.