explain how atropine mediates its pharmacological effects in the eye and the gi tract.

a 55 y o kidney transplant patient was trated with cyclosporin A to prevent organ rejection. when he developed an upper respiratory tract infection, erthyromacin was prescribed.
is there a need to change the dosage of either drug? if so which drug>
explain why and describe the likely mechanism.
a few months after the infection had cleared, the paitent who is still on cyclosporin A became depressed and took st john's wort daily on the recommendation of a freind. what potential interactions can be expected?

drug A is an anti depressant acting through serotonergic receptors. it has an oral bioA of 85% . it is extensively 90% metabolized by the liver, mainly through cyp2d6. approximately 10 % of the drug undergoes renal elimination. it has a protein binding of 75%. it has been noticed however that the clinical efficacy of drug A varies considerably in the population.
give possible non genetic reasons for the variability in clinical efficacy of drug A
what are the possible geentic polymorphisms which may affect a patient's response to drug A?
what might be the efficacy parameters you could use for studying drug A?