Mean apical concentration and duration in the comparative bioavailability of slowly absorbed and eliminated drug preparations

Therapeutic Differences in 24-h Ambulatory Blood Pressures in Patients Switched Between Bioequivalent Nifedipine Osmotic Systems With Differing Delivery Technologies

Comparing modified‐release formulations can be difficult using current bioequivalence criteria. Two 60‐mg‐once‐daily nifedipine formulations are deemed bioequivalent in Canada. This study examined the validity of the assumption that these interchangeable, but different, delivery technologies are therapeutically equivalent in maintaining systolic blood pressure (SBP) control throughout the entire dosing interval. We used 24‐h Ambulatory Blood Pressure Monitoring to objectively examine whether formulation switches changed population SBP >2 mmHg (reflecting 6% increased stroke mortality) and in what proportion of patients SBP changed ≥6 mmHg (risking unnecessary therapeutic alterations). When 20 patients, previously receiving 60‐mg‐once‐daily Nifedipine‐GITS, were switched to Mylan‐Nifedipine‐XL, population‐mean ± SE 24‐h SBP increased 3 ± 1.1 mmHg (P = 0.0173) and 8‐h nocturnal SBP increased 4 ± 1.6 mmHg (P = 0.0098). Thus, interchange of nifedipine formulations can affect therapeutic consistency. These data support existing calls to improve criteria for establishing bioequivalence between formulations employing differing modified‐release technologies.

Innovative approaches for demonstration of bioequivalence: the US FDA perspective

In this article, the authors will briefly introduce the general concepts and background of bioavailability and bioequivalence (BE), discuss the conventional method for BE demonstration, and present case examples where novel approaches have been adopted by the US FDA for BE demonstration. Here, 'novel approaches' include unconventional BE studies, as well as statistical criteria for comparison. More specifically, biowaivers, methods to demonstrate BE for highly variable drugs and drug products, and narrow therapeutic index drugs, partial AUCs as additional metrics for some modified-release drug products, methods to demonstrate BE for locally acting gastrointestinal, dermatological, nasal and inhalation products, and non-biological complex drug products, and future perspectives in the field of BE assessment will be discussed. Methods adopted by other agencies, such as European Medicines Agency and Health Canada will be compared with the FDA approaches when appropriate.

Comparative steady-state pharmacokinetic evaluation of immediate-release topiramate and USL255, a once-daily extended-release topiramate formulation

PURPOSE

Compare the pharmacokinetic (PK) profiles of immediate- and extended-release formulations of topiramate (TPM) in healthy subjects following multiple dosing, and evaluate maintenance of topiramate exposures after switching formulations.

METHODS

A randomized, open-label, single-center, two-way crossover, multiple-dose study comparing the steady-state PK profile of once-daily extended-release topiramate (USL255) to immediate-release topiramate (TPM-IR) administered twice-daily. The TPM PK profile was evaluated using standard PK parameters (e.g., AUC0-24 , Cmax , Cmin ) as well as less common PK criteria such as fluctuation index (FI), peak occupancy time (POT), and percent coefficient of variation (%CV). In addition, partial AUC (AUCp ) analyses provided comparisons of the AUC profiles over predetermined time intervals between TPM-IR and USL255. Pharmacokinetic equivalence between formulations was defined as containment of the 90% confidence intervals (CIs) of the USL255/TPM-IR geometric least-squares mean (GLSM) ratio within the equivalence limits of 80-125%. The effect of switching between treatments was assessed by evaluating equivalence of PK parameters between the day prior to formulation switch and the day immediately following formulation switch. Maintenance of steady state after switching formulations was also evaluated by comparing the slope between Cmin values at formulation switch and 24 h postswitch. Tolerability was evaluated through adverse event monitoring, vital sign measurements, and clinical laboratory evaluations.

KEY FINDINGS

USL255 was well tolerated and provided TPM plasma exposure equivalent to TPM-IR at various time intervals. USL255 also demonstrated a significantly lower Cmax (p < 0.001) and higher Cmin (p < 0.001), longer tmax , lower %CV, and 26% decreased FI, as compared with TPM-IR. Further, switching between TPM-IR and USL255 did not affect TPM concentrations, including Cmin , immediately after transitioning and at steady state.

SIGNIFICANCE

As compared with TPM-IR, USL255 provided equivalent plasma exposure with an extended absorption profile. Therefore, USL255 offers a once-daily alternative to twice-daily TPM-IR, with reduced TPM fluctuations.

Metrics for the evaluation of bioequivalence of modified-release formulations

Metrics are discussed which are used for the evaluation of bioequivalence of modified-release formulations. In order to ensure the therapeutic equivalence of the compared drug products, it would be important to contrast measures which are additional to area under the curve (AUC) and C (max). For delayed-release products, the assessment of lag times is informative. For extended-release dosage forms, comparisons of the half-value duration and the midpoint duration time are useful. For some modified-release formulations with complicated, multiphasic concentration profiles, the comparison of partial AUCs is important. In determinations of the bioequivalence of extended-release dosage forms, investigations performed under steady-state conditions rather than after single dosing can yield enhanced probability of therapeutic equivalence, especially with substantial accumulation of the drug products. In steady-state investigations of bioequivalence, evaluation of the trough concentration and of the peak trough fluctuation is informative.

Comparative pharmacokinetic analysis of USL255, a new once-daily extended-release formulation of topiramate

PURPOSE

To compare the pharmacokinetics of USL255, a once-daily extended-release (ER) formulation of topiramate (TPM), with Topamax (immediate-release TPM) in healthy subjects after oral dosing and evaluate the effect of food on USL255 bioavailability and pharmacokinetics.

METHODS

This randomized, single-center, open-label, cross-over design study had three dosing periods separated by 21 days of washout between treatments. Thirty-six volunteers received single doses of USL255 (200 mg) in fasted and fed conditions and two doses of Topamax (100 mg) administered 12 h apart. TPM plasma samples were analyzed by liquid chromatography-mass spectroscopy. Pharmacokinetic parameters were calculated by noncompartmental methods.

KEY FINDINGS

USL255 fasted pharmacokinetic parameters [point estimate (90% confidence interval, CI) compared to Topamax] were: relative bioavailability (F) 91.2% (84-99%), peak plasma concentration (C(max)) USL255/Topamax-ratio 59% (53-65%), time to reach C(max) (t(max)) 19.5 ± 7.2 h, accumulation ratio (R(ac)) 3.9 ± 1.2, effective half-life (t(1/2,eff)) 55.7 ± 19.9 h, terminal half-life (t(1/2,z)) 80.2 ± 14.2 h, and peak-occupancy-time (POT) 12.1 ± 4.0 h. Although the F and C(max) were unaffected by food, R(ac) and t(1/2,eff) increased to 4.9 ± 0.9, and 72.5 ± 15.4 h, respectively. In contrast to t(1/2,z,) t(1/2,eff) reflects absorption rate; therefore, USL255's t(1/2,eff) was significantly longer than Topamax's t(1/2,eff) (37.1 ± 6.5 h).

SIGNIFICANCE

Although bioequivalent to Topamax in extent of absorption, USL255 had a slower absorption rate as reflected in its lower C(max) and longer t(max), larger POT and longer t(1/2,eff), and similar R(ac) values to that of Topamax (q12 h). This relative flat plasma profile allows for once-daily dosing with diminished fluctuations in TPM plasma levels. In addition, neither USL255's peak nor extent of plasma exposure of TPM was affected by food.

Single-dose FTY720 pharmacokinetics, food effect, and pharmacological responses in healthy subjects

AIMS

FTY720 is a sphingosine-1-phosphate receptor agonist that redirects lymphocytes from the circulation to lymph nodes without impairing lymphocyte function. It is being developed as an immunomodulator for the prevention of acute rejection after organ transplantation. This study was performed to provide guidance on administration with respect to meals and to measure pharmacologic responses in healthy subjects.

METHODS

In this randomized, two-period, crossover study, 14 healthy subjects received placebo on day -1 of each period with baseline circadian measurements of lymphocyte count and heart rate. Subjects subsequently received a single 1 mg oral dose of FTY720 on day 1 under fasting conditions and after a high fat meal. Blood FTY720 concentrations, lymphocyte count, and supine heart rate were assessed over an 8 day period after each FTY720 dose. The effect of food on FTY720 pharmacokinetics was assessed by standard bioequivalence testing.

RESULTS

Both the peak concentration (0.65 +/- 0.17 vs 0.64 +/- 0.18 ng ml(-1)) and total exposure (AUC 149 +/- 65 vs 139 +/- 43 ng ml(-1) h) did not differ significantly between fasting and fed states, respectively. The corresponding fed/fasting ratios and 90% confidence intervals were 1.00 (0.86, 1.17) for Cmax and 0.98 (0.86, 1.11) for AUC. Under both treatment conditions peripheral blood lymphocyte count decreased from baseline by 38 +/- 9% over the first 2 days postdose and then increased towards predose values over the subsequent week. Whereas a circadian rhythm in supine heart rate was preserved in the presence of FTY720, the heart rate vs time curve was shifted downwards by 10% over the first day postdose and then recovered to prestudy values by days 3-5 postdose. These changes were asymptomatic.

CONCLUSIONS

Single 1 mg doses of FTY720 were well tolerated in healthy subjects and elicited a moderate decrease in peripheral blood lymphocyte count and a transient decrease in heart rate consistent with its pharmacological mode of action. FTY720 may be administered without regard to the timing of meals or their fat content.

Measures of exposure versus measures of rate and extent of absorption

Regulatory assessment of bioavailability and bioequivalence in the US frequently relies on measures of rate and extent of absorption. Rate of absorption is not only difficult to measure but also bears little clinical relevance. This paper proposes that measures of bioavailability and bioequivalence for drugs that achieve their therapeutic effects after entry into the systemic circulation are best expressed in terms of early [partial area under the concentration-time curve (AUC)], peak plasma or serum drug concentration and total AUC exposure for a plasma or serum concentration-time profile. With suitable documentation, these systemic exposure measures can be related to efficacy and tolerability outcomes. The early measure is recommended for an immediate release drug product where a better control of drug absorption is needed, for example to ensure rapid onset of a therapeutic effect or to avoid an adverse reaction from a fast input rate. The 3 systemic exposure measures for bioavailability and bioequivalence studies can provide critical links between product quality and clinical outcome and thereby reduce the current emphasis on rate of absorption.

Existing and new criteria for bioequivalence evaluation of new controlled release (CR) products of carbamazepine

While the three classical pharmacokinetic (PK) parameters, AUC, Cmax and tmax are adequate to assess bioequivalence of immediate release (IR) formulations, they are not designed to fully characterize the pharmacokinetic (PK) performance of controlled release (CR) formulations and provide only limited insight into the function of carbamazepine (CBZ) CR products. Thus, for reliable assessment of bioequivalence in CR formulations, there is a role for the use of additional criteria (parameters). The following are the proposed new parameters: MRT (mean residence time), Cmax/AUC, plateau time or POT (the time span associated with the concentrations within 25% of Cmax), tapical (the arithmetic mean of the times associated with POT) and Capical (the arithmetic mean of the concentrations within 25% of Cmax). The above proposed parameters, were utilized in a recent PK study of new CR products of CBZ (600 mg) designed for once daily dosing. The comparative PK analysis was conducted in a three-way crossover single dose studies of three CBZ CR formulations (Teril 600 CR tablet, CBZ 600 granulate and Timonil 600 Retard tablet). Teril 600 CR was found to be bioequivalent to Timonil 600 Retard while CBZ 600 granulate was not. This conclusion was reached utilizing both the classical and the proposed new parameters. The new parameters showed that CBZ 600 granulate has similar rate of absorption as the two 600 mg CR tablets, but its extent of absorption was lower. The new parameters examined in this paper are more attractive than the single point parameters, Cmax and tmax, for assessment of rate of absorption and the flatness of the plasma concentration versus time curve. Their potential benefit and practical utility was confirmed in this study, which demonstrated bioequivalence between a new CR and an innovator CBZ (600 mg) tablet. Absorption rate assessment is important in light of concentration-related side effects associated with CBZ therapy and the impact of fluctuations and the flatness of the CBZ plasma concentration curve on the drug efficacy and tolerability.

Criteria to assess in vivo performance and bioequivalence of generic controlled-release formulations of carbamazepine

PURPOSE

Concern persists that the criteria used to establish bioequivalence of generic drugs may not adequately guarantee the interchangeability of antiepileptic medications (AEDs), particularly controlled-release (CR) formulations. We examined the utilization of several new parameters, in addition to AUC, peak plasma concentration (Cmax), and time to reach Cmax (tmax), for the assessment of bioequivalence and in vivo performance of CBZ and other CR products. These new parameters may offer additional information for evaluation of CR products that yield a prominent plateau in the plasma time-concentration curve. They include mean residence time (MRT), Cmax/AUC, plateau time or POT (the time span associated with the concentrations within 25% of Cmax), t(apical), and C(apical) (the arithmetic mean of the POT times and concentrations within 25% of Cmax, respectively). Additional parameters for multiple-dose studies include the percentage fluctuation and the flatness of the steady state-concentration curve.

METHODS

These proposed parameters were used in two recent (single and multiple dose) two-way crossover studies of a new CR product of CBZ (Teril 400 CR) in comparison with Tegretol CR Divitab.

RESULTS

Teril 400 CR was found to be bioequivalent to Tegretol CR Divitab, by using both the classic and the additional proposed parameters. Both CBZ CR products have similar rates of absorption and similar flatness of their plasma time-concentration curves as assessed by visual inspection and the proposed parameters.

CONCLUSIONS

The additional parameters examined may supplement the traditional single-point parameters, Cmax and tmax, for assessment of rate of absorption and the flatness of the concentration curve. Their potential benefit and practical utility was confirmed in these two studies. Absorption-rate assessment is important in light of concentration-related side effects associated with CBZ therapy and the impact of fluctuations and the flatness of the CBZ plasma concentration curve on the drug efficacy and tolerability.

Criteria to assess in vivo performance of sustained release products: application to diltiazem formulations

The three classical pharmacokinetic parameters used to assess bioequivalence, AUC (total area from zero to infinity), Cmax (peak plasma concentration), and tmax (time to reach Cmax), are suitable to determine the extent and rate of absorption of immediate-release drug products. However, they may fail to evaluate the pharmacokinetic performance, particularly the rate of absorption of sustained-release (SR) formulations, which yield flat plasma curves with multiple peaks. This paper evaluates the inclusion of the following criteria for bioequivalence assessment of diltiazem SR formulations: MRT (mean arithmetic time), Cmax/AUC, peak occupancy time (POT), t(apical) (the arithmetic mean of the times associated with the concentrations within 25% of Cmax), C(apical) (the arithmetic mean of the concentration within 25% of Cmax), percent fluctuation and flatness of the curve as assessed by the coefficient of variation of the Css (steady state concentration) values obtained during a dosing interval at steady state. The above proposed criteria, as well as the classical parameters AUC, Cmax, and tmax were utilized in a recent pharmacokinetic study of a new SR product of diltiazem, Dilapress 240 (formulation A). Formulation A was analyzed following single (240 mg) and multiple (240 mg qd for 6 days) dosing at steady state (day 6) in comparison to Cardizem CD (formulation B). The bioavailability of formulation A relative to that of formulation B following single and multiple dosing was 92 +/- 28% and 90 +/- 24%, respectively. The 90% confidence intervals (Cl) over a mean AUC ratio of 89% were 78-101% (single dose, SD) and 77-101% (multiple dose, MD). following the administration of formulations A and B, identical mean values of the peak plasma concentration were obtained: 84 ng/mL (SD) and 132 ng/mL (MD). The 90% Cl over a mean Cmax ratio of 100% were 83-115% (SD) and 86-115% (MD). In the SD study, subject 8 had a relative bioavailability value of 24%, which deviated by 7.5 standard errors (SE) from the mean AUC ratio. Consequently, we repeated the single dose analysis without subject 8. The mean bioavailability data was 97 +/- 37% with a 90% Cl of 80-114% over a mean AUC ratio of 92%. ANOVA analysis did not show any formulation or period effect in all tested pharmacokinetic parameter s. On the basis of these results, these two formulations were judged to be bioequivalent. In contrast to the AUC and Cmax ratio, the 90% Cls associated with the ratio of the proposed criteria, with the exception of C(apical), did not fall within the acceptable limits. In the current study, a discrepancy was found between the above pharmacokinetic parameters, which were examined concerning their ability to detect differences in bioequivalence between SR products and the classical parameters regularly used for bioequivalence assessment. Although the parameters examined are theoretically more attractive than the single point parameters Cmax and tmax for rate of absorption assessment, their utility in bioequivalence would require further examination.

Pharmacokinetic analysis of two new sustained-release products of diltiazem designed for twice- and once-daily treatment

The pharmacokinetics of two new sustained-release (SR) products of diltiazem, Dilapress 120 mg tablets and Dilapress 240 mg tablets, was analysed and characterized in three different studies, in comparison to the following diltiazem SR formulations: Cardizem Retard, Cardizem SR, and Cardizem CD. Dilapress 120, designated for twice-daily dosing, was found to be bioequivalent to Cardizem SR and to Cardizem Retard with mean (+/- SD) relative bioavailability values of 99 +/- 27% and 113 +/- 38%, respectively. Dilapress 240, designed for once-a-day treatment, was found to have a slower absorption rate than Cardizem SR and its extent of absorption was 56 +/- 19% relative to that of Cardizem SR. However, the bioavailability of Dilapress 240 relative to that of Cardizem CD was 118 +/- 46%, indicating that the bioavailability of Cardizem CD relative to that of Cardizem SR was only 54 +/- 29%. Diltiazem is partially available due to a saturable liver first-pass effect. A high dose of Cardizem SR may partially escape this first-pass effect and, thus, achieve a higher extent of absorption than a slower SR product. Consequently, SR products of diltiazem designed for once-daily treatment may not reach the saturation stage in the liver first-pass effect process that diltiazem is susceptible to. Consequently, a twice-daily SR product of diltiazem cannot serve as a reference for extent of absorption assessments of a once-daily SR product.