The Drug development process - Pharmaceutical Biotechnology: Concepts and Aapplications - page 84

of administration. The animals are then monitored for 7-14 days, with all fatalities undergoing
extensive post-mortem analysis.
Earlier studies demanded calculation of an LD 50 value (i.e. the quantity of the drug required to
cause death of 50 per cent of the test animals). Such studies required large quantities of animals,
were expensive, and attracted much attention from animal welfare groups. Its physiological rel-
evance to humans was often also questioned. Nowadays, in most world regions, calculation of the
approximate lethal dose is suffi cient.
Chronic toxicity studies also require large numbers of animals and, in some instances, can
last for up to 2 years. Most chronic toxicity studies demand daily administration of the test drug
(parenterally for most biopharmaceuticals). Studies lasting 1-4 weeks are initially carried out in
order to, for example, assess drug levels required to induce an observable toxic effect. The main
studies are then initiated and generally involve administration of the drug at three different dos-
age levels. The highest level should ideally induce a mild but observable toxic effect, whereas the
lowest level should not induce any ill effects. The studies are normally carried out in two different
species, usually rats and dogs, and using both males and females. All animals are subjected to rou-
tine clinical examination, and periodic analyses of, for example, blood and urine are undertaken.
Extensive pathological examination of all animals is undertaken at the end of the study.
The duration of such toxicity tests varies. In the USA, the FDA usually recommends a period
of up to 2 years, whereas in Europe the recommended duration is usually much shorter. Chronic
toxicity studies of biopharmaceuticals can also be complicated by their likely stimulation of an
immune response in the recipient animals. In the context of new chemical entities (NCEs, i.e. low
molecular weight traditional chemicals), not only can the drug itself exhibit a toxic effect, but so
potentially can drug breakdown products. As proteins are degraded to amino acids, any poten-
tially toxicity associated with protein-based drugs is typically associated with the protein itself
and not degradation products.
4.13.1 Reproductive toxicity and teratogenicity
All reproductive studies entail ongoing administration of the proposed drug at three different dos-
age levels (ranging from non-toxic to slightly toxic) to different groups of the chosen target species
(usually rodents). Fertility studies aim to assess the nature of any effect of the substance on male
or female reproductive function. The drug is administered to males for at least 60 days (one full
spermatogenesis cycle). Females are dosed for at least 14 days before they are mated. Specifi c
tests carried out include assessment of male spermatogenesis and female follicular development,
as well as fertilization, implantation and early foetal development.
These reproductive toxicity studies complement teratogenicity studies, which aim to assess
whether the drug promotes any developmental abnormalities in the foetus. (A teratogen is any
substance/agent that can induce foetal developmental abnormalities. Examples include alcohol,
radiation and some viruses.) Daily doses of the drug are administered to pregnant females of at
least two species (usually rats and rabbits). The animals are sacrifi ced close to term and a full au-
topsy on the mother and foetus ensues. Post-natal toxicity evaluation often forms an extension of
such studies. This entails administration of the drug to females both during and after pregnancy,
with assessment of mother and progeny not only during pregnancy, but also during the lactation
period. Therapeutic proteins rarely display any signs of reproductive toxicity or teratogenicity.