A methionine rescue arm, which separates SAM limitation from ordinary cytotoxicity
also called PF-9366 rescue · SAM add-back
If a drug works by starving cells of a methyl donor, then adding the donor back should undo it. That single extra arm tells you whether the mechanism is what it is claimed to be.
The same thing, for a scientist
PF-9366 slows KMT2A-rearranged cells and the claimed mechanism is SAM limitation. Any antiproliferative agent produces the same readouts, so the discriminating arm is add-back: methionine or SAM supplementation alongside PF-9366, scored on the same endpoints and on global histone methylation. Rescue supports the mechanism; no rescue shows the compound is acting elsewhere and the combination results with DOT1L and PRMT5 inhibitors are additive cytotoxicity.
Design
Four arms in a KMT2A::AFF1 line and a lineage-matched non-KMT2A comparator: vehicle, PF-9366, PF-9366 plus methionine, PF-9366 plus SAM. Endpoints: proliferation, viability, and global H3K79me2, H3K4me3 and symmetric dimethylarginine by calibrated western or mass spectrometry. The comparator line matters as much as the rescue — this graph's standing objection to the MAT2A work is that MLLr-specificity was established against healthy controls rather than against another leukaemia.
What would kill it: full rescue of the methylation marks without rescue of proliferation. That would mean SAM limitation is real and is not what stops the cells growing.
- E1moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
The published MAT2A result rests on proliferation, viability, differentiation, apoptosis, cell cycling and histone methylation readouts in a model compared against healthy controls and non-MLLr lines, with no substrate add-back arm reported.
What this evidence cannot show
Abstract read; full text not read, so an add-back arm may exist unreported in it.
Evidence that argues against this (1)
- Add-back experiments with methionine are harder to interpret than they look: methionine is also the initiator of translation and a substrate for polyamine synthesis, so supplementation changes more than SAM availability. SAM itself crosses membranes poorly, which is why methionine is usually used despite being the less specific reagent. Oncofusion knowledge graph (internal analysis) 2026