One of the hazards of medicinal chemistry - or should I say, one of the hazards of long experience in medicinal chemistry - is that you start to think that you know more than you do. Specifically, after a few years and a few projects, you've seen plenty of different compounds and their activities (or lack thereof). Human brains categorize things and seek patterns, so it's only natural that you develop a mental map of the chemical space you've encountered. Problem is, any such map has to be incomplete, grievously incomplete, and if you start making too many decisions based on it (rather than on actual data), you can miss out on some very useful things.
Here's a case in point: an assay against cancer stem cells, which have been a hot research area for some time now. It may well be that some classes of tumor are initiated and then driven by such cells, in which case killing them off or inactivating them would be a very good thing indeed. This was an interesting assay, because it included control stem cells to try to differentiate between compounds that would have an effect on the neoplasm-derived cells while leaving the normal ones alone.
And what did they find? Thioridiazine is what - an old-fashioned phenothiazine antipsychotic drug. For reasons unknown, it's active against these cancer stem cells. When the authors did follow-up screening, two other compounds of this class also showed up active: fluphenazine and prochlorperazine, so I'd certainly say that this is real.
And it appears that it might actually be the compounds' activity against dopamine receptors that drives this assay. The authors found that there's a range of dopamine receptor expression in such cells, and that this correlates with the activity of the phenothiazine compounds. That's quite interesting, but it complicates life quite a bit for running assays:
Our observations of differential DR expression between normal and neoplastic patient samples strongly suggest human CSCs are heterogeneous and drug targeting should be based on molecular pathways instead of surrogate phenotypic markers.
Working out molecular pathways is hard; a lot more progress might be made at this stage of the game by running phenotypic assays - but not if they're against a heterogeneous cell population. That way lies madness.
Interesting, the phenothiazines had been reported to show some anti-cancer effects, and schizophrenic patients receiving such drugs had been reported to show lower incidences of some forms of cancer. These latest observations might well be the link between all these things, and seem to represent the only tractable small-molecule approach (so far) targeting human cancer stem cells.
But you have to cast your net wide to find such things. Dopamine receptors aren't the most obvious thing to suspect here, and ancient antipsychotics aren't the most obvious chemical matter to screen. Drop your preconceptions at the door, is my advice.