The following selleck screening library year, at the annual meeting of the American Society of Hematology, the same group reported successful treatment of four additional subjects at the highest dose. Yet the total number of people who have received this promising

experimental therapy remains very low. What are the reasons for this low access rate to a seemingly successful new therapy? From a scientific standpoint, even if men with severe haemophilia B were waiting for gene therapy in a line that stretched around the block, roughly 40% of individuals would still be excluded based on the presence of pre-existing neutralizing antibodies to AAV. Prior exposure to the wild-type virus from which the vector is engineered is ubiquitous in the population, and many individuals carry antibodies to the vector capsid. Population screening of individuals from four continents reveals that the worldwide prevalence of these antibodies is similar, and that, at least among most of the naturally occurring serotypes, the prevalence of antibodies is similar [2]. Careful studies in non-human primates, who, similar to the human population,

carry pre-existing antibodies formed in response to infection with the wild-type virus, suggest that even modest titres completely inhibit transduction when vector is delivered through the check details circulation [3]. The field has proposed a number of strategies that could be used to circumvent this obstacle [4] (Table 1). At least one of these is currently undergoing clinical investigation

[5], but proof-of-concept has not been established for any of these in subjects with severe haemophilia B. Progress in manufacture of clinical grade AAV vectors since the first human studies were conducted in the 1990s has been dramatic, and most would now agree that methodology for generation, Nintedanib (BIBF 1120) purification and characterization of recombinant AAV under current Good Manufacturing Practice (cGMP) conditions is well in hand. The products used in the clinical trials to date thus have met regulatory standards for safety, quality and consistency. However, most production methods currently in use lead to lot sizes in the range of 1–5 × 1015 vg. Since a therapeutic dose for a 70 kg man based on current studies is ~ 2 × 1012 vg kg−1, each lot is adequate to infuse 7–35 subjects, depending on yields. Thus, considerable attention is now focused on larger scale production processes.