Subject: Can we modify a drug in order to get only one of its side effects?

Can we modify a drug in order to get only one of its formerly unexpected side 
effects?
It is a known fact that Minoxidil was first used to treat hypertension but 
that it revealed to have an incidence on hair growth. Now it is used under a 
topical form in order to be effective only where applied.
In another field, I have read that in some rare cases, patients treated with 
Levobunolol for intra-ocular hypertension showed a depigmentation of the 
irides, whereas the known possible side effect of medications such as 
Levobunolol and Latanoprost is rather iridal hyperpigmentation. Is it 
imaginable to sift the properties of this kind of molecules on localized 
prostaglandin metabolism in order to get only the iris-lightening effect?
By the way, I ounce fell upon a website proposing an eye-lightening solution, 
for a huge amount of money of course, which reeks of roguery. I thought: 
either the product works and also has an effect on intra-ocular pressure, 
which makes it dangerous, or it is totally ineffective. I doubt that many 
light-eyed people would choose to get permanently dark brown eyes, but what do 
you think of the contrary? I’m sure that some pharmaceutical laboratories are 
already exploring this opportunity.
Any ideas about acting on the metabolism of melanin in eyes without damaging 
vision?

Re: Can we modify a drug in order to get only one of its side effects?