Yes, MITOMYCINS is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, T 1, O 1, M 3, Y 4, C 3, I 1, N 1, S 1.

Is it a valid word?

MITOMYCINS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a complex of antibiotic substances obtained from a streptomyces bacterium; one form (trade name Mutamycin) shows promise as an anticancer drug

Not playable in Scrabble US: MUTAMYCIN.