germany
In Germany everybody — even third parties entirely unrelated to the events or their protagonists — is obliged to help people in danger or in an emergency per §323c StGB, as long as it is not an unreasonable burden, in particular does not substantially endanger the helper, and as long as it does not conflict with other important duties. (This idea is quite foreign to common law systems where such an obligation mostly arises for people with some circumstantial or personal relation to the events.)
In Germany, this principle applies to the the doctor-patient relationship: A doctor generally has an obligation to treat any and all patients in emergencies.
I must assume that the concrete application depends on the concrete case, especially the cost/benefit calculation a qualified observer would make regarding the experimental treatment, especially compared to possible more conservative treatments. But, if we want to construct an extreme situation:
- Death is imminent and virtually ensured without treatment
- No established treatment is available
- But an experimental treatment is available
- While that treatment is still officially experimental and could normally not legally be administered outside clinical trials, it has shown great promise and on the verge of being approved.
In such a situation I would even construct an obligation to treat for a doctor. It's as if you have a rope you could throw to a drowning swimmer but you refuse to because it's just a clothesline and might break, so you stand back and do nothing instead. That seems utterly wrong.