However, if we assume it's a made-up word based on the word "mutagen," we can break it down:
* "mutagen" is pronounced MYOO-tuh-juhn
* "de-" is a prefix meaning "to remove" or "opposite of"
Based on this, a plausible pronunciation for "desmutagen" would be:
"de-MYOO-tuh-juhn"
With emphasis on the first syllable of "mutagen" and a clear distinction between the "de" prefix and the rest of the word.