Here's the latest "medical periodic table", courtesy of this useful review in Chemical Communications. Element symbols in white are known to be essential in man. The ones with a blue background are found in the structures of known drugs, the orange ones are used in diagnostics, and the green ones are medically useful radioisotopes. (The paper notes that titanium and tantalum are colored blue due to their use in implants).
I'm trying to figure out a couple of these. Xenon I've heard of as a diagnostic (hyperpolarized and used in MRI of lung capacity), but argon? (The supplementary material for the paper says that argon plasms has been used locally to control bleeding in the GI tract). And aren't there marketed drugs with a bromine atom in them somewhere? At any rate, the greyed-out elements end up that way through four routes, I think. Some of them (francium, and other high-atomic-number examples) are just too unstable (and thus impossible to obtain) for anything useful to be done with them. Others (uranium) are radioactive, but have not found a use that other radioisotopes haven't filled already. Then you have the "radioactive but toxic) category, the poster child of which is plutonium. (That said, I'm pretty sure that popular reports of its toxicity are exaggerated, but it still ain't vanilla pudding). Then you have the nonradioactive but toxic crowd - cadmium, mercury, beryllium and so on. (There's another question - aren't topical mercury-based antiseptics still used in some parts of the world? And if tantalum gets on the list for metal implants, what about mercury amalgam tooth fillings?) Finally, you have elements that are neither hot not poisonous, but that no one has been able to find any medical use for (scandium, niobium, hafnium). Scandium and beryllium, in fact, are my nominees for "lowest atomic-numbered elements that many people have never heard of", and because of nonsparking beryllium wrenches and the like, I think scandium might win out. I've never found a use for it myself, either. I have used a beryllium-copper wrench (they're not cheap) in a hydrogenation room.
The review goes on to detail the various classes of metal-containing drugs, most prominent of them being, naturally, the platinum anticancer agents. There are ruthenium complexes in the clinic in oncology, and some work has been done with osmium and iridium compounds. Ferrocenyl compounds have been tried several times over the years, often put in place of a phenyl ring, but none of them (as far as I know) have made it into the general pharmacopeia. What I didn't know what that titanocene dichloride has been into the clinic (but with disappointing results). And arsenic compounds have a long (though narrow) history in medicinal chemistry, but have recently made something of a comeback. The thioredoxin pathway seems to be a good fit for exotic elements - there's a gadolinium compound in development, and probably a dozen other metals have shown activity of one kind or another, both in oncology and against things like malaria parasites.
Many of these targets, though, are in sort of a "weirdo metal" category in the minds of most medicinal chemists, and that might not reflect reality very well. There's no reason why metal complexes wouldn't be able to inhibit more traditional drug targets as well, but that brings up another concern. For example, there have been several reports of rhodium, iridium, ruthenium, and osmium compounds as kinase inhibitors, but I've never quite been able to see the point of them, since you can generally get some sort of kinase inhibitor profile without getting that exotic. But what about the targets where we don't have a lot of chemical matter - protein/protein interactions, for example? Who's to say that metal-containing compounds wouldn't work there? But I doubt if that's been investigated to any extent at all - not many companies have such things in their compound collections, and it still might turn out to be a wild metallic goose chase to even look. No one knows, and I wonder how long it might be before anyone finds out.
In general, I don't think anyone has a feel for how such compounds behave in PK and tox. Actually "in general" might not even be an applicable term, since the number and types of metal complexes are so numerous. Generalization would probably be dangerous, even if our base of knowledge weren't so sparse, which sends you right back into the case-by-case wilderness. That's why a metal-containing compound, at almost any biopharma company, would be met with the sort of raised eyebrow that Mr. Spock used to give Captain Kirk. What shots these things have at becoming drugs will be in nothing-else-works areas (like oncology, or perhaps gram-negative antibiotics), or against exotic mechanisms in other diseases. And that second category, as mentioned above, will be hard to get off the ground, since almost no one tests such compounds, and you don't find what you don't test.