Phthalazinediones that function as intracellular redox modulators are useful in treating cells in various disease states where intracellular redox status is impaired. By buffering aberrant redox states, phthalazinediones enable cellular processes essential for survival and augment medical treatments. The phthalazinediones of the invention can modulate functions related to cell growth, differentiation, activity, or death, to correct aberrations and restore homeostasis, and can serve as adjunctive therapy in treating various disease conditions.