Exogenous drugs

The amount of exogenous drug is difficult to determine. Before conducting release experiments, loaded halloysite was always washed with a large amount of water during a 5 minute rinse to ensure removal of material from the surface. However, we cannot exclude some of the active agent being attached to the outer surface of the halloysite, especially in the natural gap-defects on the cylinder surface at the end of the rolled clay sheet (it is a natural pocket on the tubule surface). The typical 5-10 % initial release burst we observed may be related to dissolution of this material. 

A ligand is a chemical messenger able to bind to a receptor. May be endogenous or exogenous (drugs). 

Hormone substitution is sometimes referred to as simple replacement therapy. However, the use of exogenous hormones to replace normal endocrine function can be a complicated task. Problems such as regulation of optimal dosage, the interaction of the exogenous drug with other endogenous hormone systems, and drug-induced side effects are frequendy encountered. 

The critical element for differentiating between endogenous and exogenous drugs is the distribution of drugs between the semiaccessible and inaccessible domains. This distribution is quantitatively defined by the extended safety zone ratio (Resz) ... 

So from a simple mass transport point of view, hair is less susceptible to contamination by exogenous mechanisms than urine by endogenous mechanisms. But then hair, but not urine, is cleansed by normal hygienic practices and by special laboratory wash procedures. The latter are further strengthened by special kinetic analysis of the wash data. And, finally, the deposition of exogenous drugs onto hair, unlike their accumulation in urine, is not associated with the formation of metabolites. 

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