many drug patents have recently expired or are scheduled to expire in the near future . in response , many drug manufacturers have expanded their generic drug portfolio , which requires them to conduct clinical trials that demonstrate that their generic equivalents perform similarly to the innovator drug product .
regulations introduced by the united states food and drug administration ( fda ) and the european medicines agency ( ema ) over the last thirty - five years have strengthened measures to ensure the bioequivalency of drug products , which may be simultaneously manufactured by multiple drug makers [ 3 - 5 ] .
bioequivalence and bioavailability testing standards have also emerged following recognition that bioinequivalence and variations in the bioavailability of drug products can result in therapeutic failure and/or toxicity [ 6 - 8 ] . in the united states ,
the successful approval of new and abbreviated new drug applications requires regulatory approval by the fda .
recent studies have suggested that this process takes nearly a decade to complete the required series of pre - clinical studies and clinical trials .
drug development is an expensive process that is marked by a high - failure rate . for these reasons ,
early stage bioequivalence and pharmacokinetic studies are essential in determining the fate of new drug products . in this study
, we sought to systematically assess the current trends of ongoing and recently completed bioequivalence and bioavailability trials that have been registered within a national clinical trials registry .
this study provides insight regarding the characteristics of current bioequivalence and bioavailability trials and may also provide assistance in prioritizing future areas of research . ]
we identified bioequivalence and bioavailability trials registered in clinicaltrials.gov using the key words  bioequivalence  and  bioavailability  .
briefly , clinicaltrials.gov is a publicly - available registry of clinical research studies that is maintained by the u.s . national institutes of health . as of mid-2013
there were nearly 150,000 studies registered in clinicaltrials.gov , with study sites in 185 countries ( http://www.clinicaltrials.gov ) .
our search was restricted to identify studies registered between 01 october 2007 and 31 december 2012 to coincide with the enactment of a federal law in 2007 that mandated the registration of all phase 2 - 4 interventional trials involving drugs , biological agents , and medical devices .
we excluded all observational trials ( n=34 ) as well as trials that were  suspended 
( n=7 ) ,  terminated  ( n=33 ) , or  withdrawn  ( n=26 ) .
the remaining trial registry entries were systematically examined and the following data elements were extracted : a unique trial identifier , study title , recruitment status , phase ( 0 - 4 ) , study design , blinding status , interventional assignment to trial arms , primary endpoint classification , primary purpose of the trial , age group and gender eligibility criteria , and anticipated enrollment size .
descriptive statistics were used to characterize the bioequivalence and bioavailability trials identified in the clinicaltrials.gov registry .
comparisons between ongoing trials and those that have been completed were performed using the -test or fisher s exact test .
all statistical analyses were undertaken in stata 11.2 ( statacorp lp , college station , tx , usa ) .
we identified bioequivalence and bioavailability trials registered in clinicaltrials.gov using the key words  bioequivalence  and  bioavailability  .
briefly , clinicaltrials.gov is a publicly - available registry of clinical research studies that is maintained by the u.s . national institutes of health . as of mid-2013
there were nearly 150,000 studies registered in clinicaltrials.gov , with study sites in 185 countries ( http://www.clinicaltrials.gov ) .
our search was restricted to identify studies registered between 01 october 2007 and 31 december 2012 to coincide with the enactment of a federal law in 2007 that mandated the registration of all phase 2 - 4 interventional trials involving drugs , biological agents , and medical devices .
we excluded all observational trials ( n=34 ) as well as trials that were  suspended 
( n=7 ) ,  terminated  ( n=33 ) , or  withdrawn  ( n=26 ) .
the remaining trial registry entries were systematically examined and the following data elements were extracted : a unique trial identifier , study title , recruitment status , phase ( 0 - 4 ) , study design , blinding status , interventional assignment to trial arms , primary endpoint classification , primary purpose of the trial , age group and gender eligibility criteria , and anticipated enrollment size .
descriptive statistics were used to characterize the bioequivalence and bioavailability trials identified in the clinicaltrials.gov registry .
comparisons between ongoing trials and those that have been completed were performed using the -test or fisher s exact test .
all statistical analyses were undertaken in stata 11.2 ( statacorp lp , college station , tx , usa ) .
from october 2007 through december 2012 there were 2,388 interventional bioequivalence and bioavailability trials registered in clinicaltrials.gov .
of these , 227 ( 10% ) trials are actively recruiting participants , 15 ( 1% ) are recruiting by invitation only , 87 ( 4% ) are engaged in data analysis , and 2,059 ( 86% ) have been completed .
ongoing bioequivalence and bioavailability trials are more likely to be in later phase clinical trials , as reflected by a decrease in the proportion of phase 0 , 1 , and 1/2 trials from 75% among completed studies to 36% of ongoing studies ( p<0.001 ) .
ongoing trials are also more likely to be double - blinded ( 27% vs. 12% ; p<0.001 ) and have larger sample sizes ( p<0.001 ) .
similarly , ongoing trials are more likely to feature parallel group assignment and less likely to be crossover trials ( p<0.001 for both ) .
there has also been an increase in the proportion of trials that primarily involved research on treatments from 42% to 55% ( p<0.001 ) .
the proportion of trials that exclusively recruited male participants declined from 20% to 9% ( p<0.001 ) and the number of trials that enrolled children increased from 3% to 17% ( p<0.001 ) .
more than half of the bioequivalence and bioavailability trials registered in clinicaltrials.gov were conducted internationally ( 58% ) . among ongoing trials ,
the majority of ongoing trials are recruiting participants in north america ( 52% ) and europe ( 26% ) ; however , east asia ( 7% ) , the middle east ( 4% ) , and south america ( 4% ) are also involved in several ongoing bioequivalence and bioavailability trials .
from october 2007 through december 2012 there were 2,388 interventional bioequivalence and bioavailability trials registered in clinicaltrials.gov .
of these , 227 ( 10% ) trials are actively recruiting participants , 15 ( 1% ) are recruiting by invitation only , 87 ( 4% ) are engaged in data analysis , and 2,059 ( 86% ) have been completed .
ongoing bioequivalence and bioavailability trials are more likely to be in later phase clinical trials , as reflected by a decrease in the proportion of phase 0 , 1 , and 1/2 trials from 75% among completed studies to 36% of ongoing studies ( p<0.001 ) .
ongoing trials are also more likely to be double - blinded ( 27% vs. 12% ; p<0.001 ) and have larger sample sizes ( p<0.001 ) .
similarly , ongoing trials are more likely to feature parallel group assignment and less likely to be crossover trials ( p<0.001 for both ) .
there has also been an increase in the proportion of trials that primarily involved research on treatments from 42% to 55% ( p<0.001 ) .
the proportion of trials that exclusively recruited male participants declined from 20% to 9% ( p<0.001 ) and the number of trials that enrolled children increased from 3% to 17% ( p<0.001 ) .
more than half of the bioequivalence and bioavailability trials registered in clinicaltrials.gov were conducted internationally ( 58% ) . among ongoing trials ,
the majority of ongoing trials are recruiting participants in north america ( 52% ) and europe ( 26% ) ; however , east asia ( 7% ) , the middle east ( 4% ) , and south america ( 4% ) are also involved in several ongoing bioequivalence and bioavailability trials .
this study reveals that bioequivalence and bioavailability trials are part of a global clinical research enterprise . when compared to completed trials , ongoing trials are in later phases of clinical development , recruiting larger numbers of participants , and more likely to recruit women and children .
these data suggest that bioequivalence and bioavailability studies are undergoing a transformation as drug makers seek to characterize the safety and efficacy of drug products in more rigorous trials that closely resemble their anticipated patient population .
as the costs of healthcare and drug development have risen , there is a mounting incentive for improving our understanding of existing treatments while also enabling breakthrough discoveries .
recently , the united kingdom has attempted to strategically align their clinical research funding with their public health priorities .
although similar measures have not been enacted in the united states , it behooves policy makers to consider the vital role that bioequivalence and bioavailability studies play in bringing new and generic drug products to the public .
as noted here , the quality of bioequivalence and bioavailability studies has improved rapidly , even over the last five years , and the horizon is bright . however , as the national debate on healthcare reform and research priorities unfolds we may need to re - evaluate our approach to bioequivalence and bioavailability trials to ensure that safe and efficacious medicines swiftly reach healthcare providers and their patients .
 drug development is an expensive process that is marked by a high - failure rate . for this reason 
 early stage bioequivalence and pharmacokinetic studies are essential in determining the fate of new drug products . in this study , we sought to systematically assess the current trends of ongoing and recently completed bioequivalence and bioavailability trials that have been registered within a national clinical trials registry . 
 all bioequivalence and bioavailability studies registered in the united states clinicaltrials.gov registry from late-2007 through 2011 were identified . over this period , 
 more than 2300 interventional bioequivalence and bioavailability trials were registered . as of 2013 , 
 the vast majority of studies ( 86% ) have been completed , 10% are actively recruiting participants , and the remainder are engaged in data analysis ( 4% ) . when compared to completed trials , ongoing trials are in later phases of clinical development , recruiting larger numbers of participants , and more likely to recruit women and children ( p<0.001 for all ) . 
 these data suggest that the quality of bioequivalence and bioavailability studies has improved rapidly , even over the last five years . 
 however , further work is needed to sustain  and accelerate  these improvements in the design of bioequivalence and bioavailability studies to ensure that safe and efficacious medicines swiftly reach healthcare providers and their patients . 
Introduction
Methods
Selection of bioequivalence and bioavailability trials
Statistical analyses
Results
Trial characteristics
Geographic distribution
Discussion