Check the solubilities of your compounds in other solvents and surfactants like DMSO, ethanol or tween. Also cells can tolerate low concentrations of methanol so it depends how soluble your compound is in methanol. Still with TPO you can use tween-20 to make an emulsion or disolve in DMSO.

(Article Cytotoxicity, Colour Stability and Dimensional Accuracy of 3...

Dissolving photoinitiators like TPO (Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide) and Irgacure 2959 (also known as Irgacure 2959 or Darocur 2959, which is 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one) for cytotoxicity assays can be a critical step to ensure that the solvent itself doesn't introduce any unwanted cytotoxic effects. Using toxic solvents like methanol should be avoided in this case.

Here are some alternative solvent options and steps to consider for dissolving these photoinitiators:

All the best