RESUMEN

'Highly variable drugs' have been defined as those drugs for which the within-subject variability (WSV) equals or exceeds 30% of the maximum concentration (Cmax) and/or the area under the concentration versus time curve (AUC). Despite the fact that highly variable drugs are generally safe with flat dose response curves, the bioequivalence of their formulations is a problem because the high variability means that large numbers of subjects are required to give adequate statistical power. Highly variable drug products are poor quality formulations where high within-formulation variability (e.g. tablet to tablet variability) poses a problem rather than high innate WSV of the drug itself. A further problem caused by high variability is that a subset of the population may respond differently to the two formulations producing a significant subject x formulation interaction. Practical examples are shown using replicate designs. The methods proposed to deal with the problems posed by highly variable drugs include: (i) Drug regulatory jurisdictions states that the 90% confidence interval (90% CI) around the test to reference geometric mean ratio (GMR) is required to fit with bioequivalence acceptance limits of 0.8 - 1.25 for both Cmax and AUC. The WSV for single point estimation of Cmax is often greater than that for AUC. One strategy therefore is not to require a 90% CI for Cmax of drugs that do not exhibit a toxicity associated with Cmax and merely require the GMR to fall within the acceptance limits. (ii) To arbitrarily broaden the bioequivalence acceptance limits. For example, to permit a sponsor to justify the use of wider limits e.g the 90% CI around the GMR of Cmax values might be required to fit within acceptance limits of 0.75 - 1.33 or even 0.70 - 1.42. (iii) A more systematic approach would be to broaden the acceptance limits by scaling to either the residual variance from a 2-period design or to the WSV of the reference product in a replicate design. Subsequent evaluations of scaling procedures have demonstrated that smaller numbers of subjects are required for bioequivalence studies on formulations of highly variable drugs. A disadvantage of scaling is that the method is less sensitive to differences between the means compared with unscaled treatment, such that the GMR may prove to be unacceptably low or high. This possibility has let to a suggestion that the GMR must fall within acceptance limits of 0.8 - 1.25 in scaled treatments. (iv) A similar method is to scale the metric rather than the acceptance limits. This method was proposed by the United States' Food and Drug Administration in the context of Individual bioequivalence, but may also be applied (v) to average bioequivalence. (vi) To carry out bioequivalence studies at steady state whenever a multiple dose regimen is ethically acceptable for healthy volunteers. This solution is based on the observation that high variability in a single dose study tends to be dampened at steady state, thus increasing statistical power. Drug regulators have not favored this approach on the grounds that bioequivalence testing should be based on the most discriminating test possible. (vii) Finally the use of metabolite data has been proposed since in many (but by no means all) cases, metabolite is less highly variable than that of the parent drug. This subject remains controversial except when the administered substance is a prodrug which converted by metabolism into the active drug.

Asunto(s)

RESUMEN

OBJECTIVE: To examine the effect of the exposure measures C(max) (peak exposure), AUC(E) (early exposure) and AUC (total exposure) on the bioequivalence of two sustained release formulations of bupropion (i) in the fasted state and (ii), after a high fat meal. The ratio C(max)/AUC (sensitive to rate of absorption) was also evaluated. METHODS: A two-formulation, two-sequence, four-period replicate design study was performed in 29 healthy men and women after an overnight fast. Similarly, a two-period study was performed in 20 healthy men and women after ingestion of a high fat breakfast. Plasma concentrations of bupropion were measured by HPLC/MS/MS and the data were analyzed (SAS PROC MIXED) by the Schuirmann-Sattersthwaite procedure (four-period study) and by the two one-sided test procedure (SAS PROC GLM) (two-period study). Standard bioequivalence limits of 80 - 125% were applied to all measures including AUC(E) and C(max)/AUC. RESULTS: In the fasting study, the mean plasma concentration vs. time curves from (including over the first 24 hours) following the two administrations of each formulation were similar although there was a significant difference in median t(max) between formulations. This may have contributed to a low estimate of geometric mean ratio (GMR) for AUC(E) (69%) which was judged to have failed bioequivalence. There was also rather low GMRs for Cmax (88%) and C(max)/AUC (89%) but these measure passed because the within-subject variabilities (WSV) were relatively low (19.6% and 11.2%, respectively). Total exposure (AUC(last)) met standard bioequivalence limits of 80 - 125% easily. The raw data from the two-period fed also showed differences in the shapes of the plasma concentration vs. time curves around C(max) although there was no difference in median t(max). The WSV at median t(max) was high (34%) as was the GMR (117%) for AUCE which failed, as did C(max) (GMR 112%). The WSV was very high at early time points before settling into a "plateau" at about 11%. DISCUSSION: There was no "spike" in the plasma concentration vs. time profiles up to median t(max) or beyond and therefore there was no evidence of dose dumping of the test formulation in either fasted or fed states. No bioequivalence limits have been set for AUC(E) but the application of standard BE limits of 80 to 125% meant that the fed study was clearly underpowered given the high WSV at early time points. CONCLUSIONS: More research is needed on the interesting concept of early exposure. The WSV is often high at median t(max) which means that standard bioequivalence limits of 80 - 125% may be inappropriate. Despite the lack of dose dumping, application of AUC(E) to the fasting study, would have resulted in failure to declare bioequivalence since the GMR for this measure was only 69.5%. Application of a 90% confidence interval to AUC(E) to the fed study would have required powering to cope with the fact that this measure was highly variable.

Asunto(s)

RESUMEN

PURPOSE: To test the hypothesis that the pharmacokinetics of d-methylphenidate (d-MPH) would be altered by food ingested before administration of an immediate release formulation (dl-MPH- IR) but not when food is ingested before a slow release formulation (dl-MPH-SR). METHODS: A randomized, four-phase, open label, crossover design was conducted in 24 healthy men who each received, on separate occasions, dl-MPH-IR and dl-MPH-SR taken after an overnight fast and 15 min after a standardized breakfast (20% protein, 21% fat, 59% carbohydrate). Plasma MPH levels were monitored by a validated, stereoselective. GLC-ECD method. RESULTS: For plasma d-MPH, there were significant differences (ANOVA) between dl-MPH-IR and dl-MPH-SR in tmax, Cmax (peak exposure), and Cmax/AUC (sensitive to rate of absorption). Dl-MPH-SR on average delayed tmax from 2.3 to 3.7 h and lowered Cmax 34%. There was no significant difference between the formulations in AUC (extent of absorption). For dl-MPH-IR, food significantly increased Cmax (23%) and AUC (15%) and for dl-MPH-SR the corresponding increases were Cmax (17%) and AUC (14%). After dl-MPH-IR, food delayed average tmax from 2.0 to 2.5 but had no effect on tmax after dl-MPH-SR. There was no effect of food on Cmax/AUC (rate of absorption). CONCLUSIONS: Food caused a significant increase in extent of absorption but had no effect on rate of absorption of d-MPH after either dl-MPHIR or dl-MPH-SR.

Asunto(s)

RESUMEN

PURPOSE: To establish procedures for the effective evaluation of bioequivalence (BE) for highly-variable drugs and drug products (HVD/P). METHODS: 2- and 4-period crossover BE studies with 24 subjects were simulated which generally assumed within-subject coefficients of variation of 40%. The relationship between the fraction of studies in which BE was accepted (the statistical power) and the ratio of geometric means (GMR) of the two formulations was evaluated for various methods of analysis. These included, primarily, scaled average BE (ABE), the corresponding approach of expanding BE limits (BEL), and, for comparison, unscaled ABE and scaled individual BE (IBE). RESULTS: Scaled ABE and expanding BEL showed very similar properties in both 2- and 4-period studies. They had steeper power curves than scaled IBE. Unscaled ABE had very low statistical power. The acceptance of BE by unscaled and scaled ABE and expanding BEL was almost independent of subject-by-formulation interaction and the ratio of within-subject variations of the two formulations. By contrast, the conclusions reached by scaled IBE were strongly affected by these parameters. CONCLUSIONS: Scaled ABE and expanding BEL evaluate BE effectively for HVD/P in both 2- and 4-period investigations. However, additional, useful information can be obtained from 4-period studies.

Asunto(s)

Asunto(s)

Asunto(s)

RESUMEN

At the AAPS/FDS Workshop (Crystal City, Arlington, VA, March 6-8, 1995), it was agreed that some form of scaling should be permitted for highly variable drugs, although there was no agreement on a method. Currently, much emphasis is focused on developing a practical methodology for individual bioequivalence (IBE) and population bioequivalence (PBE) to replace or complement average bioequivalence (ABE). The latter requires only the mean bioavailabilities of two formulations to be sufficiently similar, whereas PBE also considers their distributions. IBE, on the other hand, is a comparison of the individual responses to the two formulations within subjects and is therefore concerned with switchability (interchangeability) between two multisource formulations. Multisource formulations refer (i) to generic copies of an innovator's formulation or (ii) to different formulations used in stages leading up to the final marked formulation. Evaluation of both PBE and IBE require replicate design studies. The FDA Working Group on IBE has been experimenting with methods in which a one-sided 95% confidence interval is computed based on the Bootstrap technique which ensures that the consumer risk is maintained at 5%. The IBE metric can then be scaled according to the within subject variance of the reference formulation. Thus if the variability of the test formulation (T) is higher than that of the reference (R), the formulation may fail IBE but not ABE. Conversely, if R is more variable than T, then the formulations may be considered to be IBE, even with a difference in means of more than 20%. Experimentation with existing data on our files shows that scaling has a considerable effect on the IBE decision for highly variable drugs. Evidence will also be presented to show that scaling makes the conditions more conservative for potent drugs with steep dose response curves reducing the risk of two generic formulations being BE with the same reference product but not BE with each other. On the other hand, broadening the BE limits for safe, highly variable drugs increases statistical power and reduces the number of subjects required. Even with the introduction of scaling, however, it is clearly difficult to obtain a single IBE criterion suitable to be applied to all drug products/studies.

Asunto(s)

RESUMEN

BACKGROUND: Amoxapine is a dibenzoxazepine type tricyclic antidepressant. The mechanism of clinical action in patients is not well understood. In animals, amoxapine blocks the reuptake of norepinephrine and, to a lesser extent serotonin, into their respective neurons and blocks the response of dopaminergic receptors to dopamine. The major metabolite, 8-hydroxyamoxapine, has similar norepinephrine uptake inhibitory action to the parent drug, but has a more pronounced inhibitory action on serotonin uptake. Another major metabolite, 7-hydroxyamoxapine blocks post-synaptic dopamine receptors. SUBJECTS AND METHODS: The present study was a traditional two-treatment, two-period, two-sequence crossover design with the primary objective to investigate the average bioequivalence of two formulations of amoxapine. Secondary objectives were to explore the potential roles of metabolites and truncated (partial) areas in bioequivalence studies. Serial plasma samples were harvested from immediately pre dose to 96 hours post dose. The parent drug and the two hydroxy metabolites were monitored by validated HPLC procedures. Geometric mean ratios and 90% confidence intervals (90% CIs) were calculated for Cmax, AUCinfinity, the truncated areas of AUC, AUCinfinity/Cmax, and the truncated areas of AUC/Cmax. RESULTS: The results indicated that the two formulations were bioequivalent in terms of the conventional parameters Cmax and AUC for all three analytes in the sense that the 90% CIs fitted entirely within preset bioequivalence limits of 80-125%. Moreover, the 90% CIs for the truncated areas AUC2.0hr through AUClast and Cmax/AUC1.0hr through Cmax/AUClast of all three analytes also fell entirely within bioequivalence limits of 80-125%. It was concluded that it was unnecessary to have harvested plasma samples beyond 12 hours, in which case additional plasma samples could have been devoted to the more precise estimation of tmax and Cmax. CONCLUSION: Of the three analytes, test and reference individual plasma concentration versus time curves of 8-hydroxyamoxapine were more closely superimposable than those of the other two analytes. Any decision to use metabolite data in bioequivalence studies, however, must be made a priori to avoid introduction of bias arising from selective post hoc manipulation of the raw data; and to facilitate the design of blood sampling schedules based on prior information about the tmax of the selected analyte.

Asunto(s)

RESUMEN

Linked administrative health care utilization databases offer potential benefits for postmarketing surveillance. The value of the Saskatchewan datafiles in an acute adverse event signalling scheme has been evaluated using two benzodiazepines. The first 20,000 patients dispensed lorazepam and the first 8525 patients dispensed alprazolam were followed through the datafiles over the year after their initial prescription of the relevant drug, and all medical services occurring during treatment were recorded. The most frequent adverse drug reactions to benzodiazepines are drowsiness, depression, impaired intellectual function and memory, lethargy, impaired coordination, dizziness, nausea and/or vomiting, skin rash, and respiratory disturbance. Data from our study showed that sleep disorders, depressive disorders, dizziness and/or vertigo, respiratory symptoms, esophagus and stomach disorders, and inflammatory skin conditions occurred significantly more often in the first 30 days after the initial prescription than in the succeeding six months in both drug groups, indicating that they are important adverse events. There are several limitations to the methodology; however, the results of the analysis indicate that the use of administrative health care utilization datafiles in a systematic assessment to signal potential acute adverse drug reactions is a feasible proposition, but further studies are required to assess whether events are real adverse reactions.

Asunto(s)

RESUMEN

This study demonstrated that the mere fact that two multisource drug formulations are bioequivalent with the same reference formulation does not guarantee that they are bioequivalent with each other. The present unscaled bioequivalence limits (BEL) of 0.80 to 1.25 permitted far greater deviation from unity of the geometric mean ratio (GMR) for multisource formulations with low within-subject variabilities than for drugs with high variabilities. Scaling the BEL drastically reduced the maximum deviation from unity of GMRs for two multisource formulations each bioequivalent with the same reference product while broadening the BEL for highly variable drugs. It was concluded that scaling was consistent with the principle of switchability for toxic drugs with low variability and for safe, highly variable drugs. On the other hand, scaling need not be applied to safe drugs with low variability and should not be applied in the unusual case of a highly variable drug with a narrow therapeutic range.

Asunto(s)

Asunto(s)

Asunto(s)

Asunto(s)

RESUMEN

In two experiments, the capacity of Parkinson patients and normal controls to generate letters randomly was investigated using a dual task. The task was based on Baddeley's work (Baddeley, A. D., Quarterly Journal of experimental Psychology 18, 119-129, 1966) where participants vocalize letters of the alphabet in random order. The task was performed under two conditions where letters were (a) vocalized at a specified rate (0.5, 1, 2 and 4 sec); and (B) letters were vocalized at the same rates but with a secondary task of sorting cards into one, two, four and eight categories. The aim was to test the notion of non-specific slowing in response selection processing in Parkinson's disease. Results showed that randomness decreased as generation rate increased in both groups, but with the patient group significantly impaired in comparison to controls. The notion of a limited capacity response selector based on disruption to the Supervisory Attentional System is invoked to account for the impaired capacity of the patient group to generate information internally.

Asunto(s)

Asunto(s)

Asunto(s)