On a positive note, the modifications have not been associated with any adverse events and, in animal studies, have been associated

with reduced levels of antibody production. Indeed, it has previously been observed that PEGylation of molecules tends to reduce immunogenicity [113]. However, the combination with other molecules has produced some alterations in the way the FVIII behaves in standard assays of FVIII activity. Whilst this has sometimes been similar to the discrepancies already seen with concentrates and B-domain-deleted products, the discrepancies are somewhat reagent- and product-dependent which may result in problems limiting selleck chemicals their widespread use [109]. The initial trials with PEGylated FVIII molecules have not demonstrated any adverse events of concern, but these were single injections into PTPs. Use of PEG in other circumstances has generally been for limited courses of treatment. There is therefore a possible concern regarding PEG accumulation when patients receive repeated injections for many years. This is particularly relevant in view of the fact that PEG is a non-physiological molecule with no specific degradation or excretion pathway [113]. In general it appears that, after administration, PEG is either taken up by macrophages or follows the pathway of the molecule to which it has been BMN 673 purchase linked. In very high doses over

periods of 6–12 months, modest degrees of vacuolation have been seen in animal models. It has been calculated that for some of the modified FVIII Forskolin in vitro molecules described above, a 60 IU kg−1 dose will constitute a 4 μg kg−1 load of PEG which

equates to approximately 20 mg per year for an adult on prophylaxis. However, the doses required to produce the vacuolation changes noted above are approximately four orders of magnitude greater than this, and in any event were not associated with any functional toxicity. Large doses of other PEGylated molecules have been given over 6-month periods without any toxicity. Moreover, although there is no specific pathway, it appears that PEG can be broken down by cytochrome enzymes and some of it will appear in the urine [113]. Overall, it seems unlikely that significant PEG toxicity will be encountered using these modified FVIII molecules but it will undoubtedly be a subject for postmarketing monitoring. The relatively modest prolongation of half-life achieved with the agents to date means that it will be necessary to prioritize frequency, trough levels and costs when they become available for use. In current prophylactic regimens the principal concern centres on the trough level of FVIII. In particular, the critical factor in reducing bleed frequency is the time spent below 0.01 IU mL−1, which is known to be a major determinant of bleed frequency [114]. The peak level achieved may also be important for individuals taking part in physical activity where a period of higher FVIII level is desirable.