The idea here is to develop a biocompatible proto-drug, that once ingested, can last in the body for a certain period of time without any toxicity, and upon exposure to the sunlight, can protect against ultraviolet light, especially the light in the UVB range.
Some key points from a first principles approach:
-Present-day sunscreen products are useful by the virtue of two categories of ingredients: the physical barrier compounds (Zinc Oxide, for example), and the chemical dampeners (aminobenzoic acid, avobenzone, etc). Both of these categories basically don't let the UVB light reach the dermis layer of skin either by physically reflecting it away or by absorbing it. Hence, the idea here is to come up with an edible/ingestable version of a precursor molecule that could be taken as a drug, and only after we come in contact with the sunlight the precursor changes into the actual 'blocker/dampener' molecule. Think of this final molecule as a Zinc oxide analog. In principle, biology allows such a conversion because this is exactly how Vitamin D is activated in our body.
-How to approach the design of this drug? To start with, since the drug (final molecule) is expected to work as a sunscreen, the most important feature of the drug should be its tissue specific. The protodrug should effectively localise in the skin cells. We might leverage the melatonin molecule that is unique to skin cell, and design the protodrug for melatonin-tropism.
(The idea, I concede, is kind of crazy, but at least should work in theory!)