Who would sign up for your study? The diligent person already on truvada - would they risk getting placebo? No. Or the person too careless to take truvada? Would they go to the trouble of participating in a study for a 50% chance of protection? No.
Ethics and managing relationships with the public overlap.
But I don't think the particular aspect of "making sure you have enough participants" interacts very much with ethics. Especially when the failure state in question is an honest failure to attract them.
