Why isn’t there an endpoint for measuring polyglucosan bodies present in human tissue? Isn’t it a measurable quantity and important for potential drug use in pre-symptomatic patients?

Biochemical markers are not acceptable end points because in certain diseases fixing biochemical defects do not really improve the symptoms. Therefore endpoints are all physical measures like 6 min walk test, feeling better or improving urinary incontinence.
It is even harder to include non-symptomatic patients in clinical trials.