Cell-impermeant pyridinium derivatives of psoralens as inhibitors of keratinocyte growth11Abbreviations: BQ, 1,4-benzoquinone; DEPC, diethyl pyrocarbonate; DMEM, Dulbecco’s Modified Eagle Medium; EGF, epidermal growth factor; FADU, fluorescent analysis of DNA unwinding; 4′-Pyr-H2TMP, 4′-pyridinium-4′,5′-dihydro-4,4′,8-trimethylpsoralen; 5′-Pyr-H2TMP, 5′-pyridinium-4′,5′-dihydro-4,5′,8-trimethylpsoralen; H2TMP, 4′,5′-dihydro-4,5′,8-trimethylpsoralen; IFN-γ, interferon-γ; NOS2, inducible nitric oxide synthase; LB, Luria-Bertani broth; PUVA, psoralen plus UVA light; RT-PCR, reverse transcription-polymerase chain reaction; TdR, thymidine; TMP, 4,5′,8-trimethylpsoralen; and UVA light, ultraviolet light in the range of 320–400 nm.