Purpose　Photodynamic　therapy　(PDT)　schedules　are　based　on　sensitiser　dose,　light　dose,　and　drug-light　interval.　The　aim　of　the　phase　Iota　study　was　to　choose　optimal　dose　and　drug-light　interval　for　PDT　with　photocyanine　using　pharmacokinetics　(PK)　and　pharmacodynamics　(PD).　Methods　Twenty-eight　cancer　patients　were　enrolled.　In　trial　A,　12　patients　received　one　of　four　ascending　doses　of　photocyanine　intravenously　24　h　prior　to　180-270　J/cm(2)illumination.　0.2　mg/kg　dose　was　infused　to　ten　patients　12-48　h　prior　to　120　J/cm(2)illumination　in　trial　B.　In　trial　C,　0.1　mg/kg　dose　was　infused　to　six　patients　6　or　12　h　prior　to　180-270　J/cm(2)illumination.　Serum　concentrations　of　photocyanine　were　measured,　and　simulations　were　performed　to　assess　the　effect　of　drug　exposure　in　tissue　on　responses.　Results　Analysis　of　photocyanine　levels　of　patients　indicated　that　the　two-compartment　model　best　fit　the　data.　Simulations　showed　that　the　rates　of　the　drug　entering　tissues　and　leaving　tissues　were　equal　at　8-12　h　after　injection.　Patients　experienced　pain　which　was　related　to　photocyanine　serum　levels,　especially　with　serum　levels　above　2500　ng/ml.　Fewer　non-responders　were　observed　at　serum　levels　higher　than　1000　ng/ml　for　illumination　at　least　12　h　after　administration.　Conclusion　It　is　the　first　report　of　human　trials　of　photocyanine,　and　the　results　suggested　that　patients　receive　180　J/cm(2)illumination　about　20-30　min　at　serum　concentrations　of　photocyanine　between　1000　and　2500　ng/ml　at　least　10　h　after　administration.