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Notes: Abstract Purpose: Pc4 is a silicone phthalocyanine photosensitizing agent that is entering clinical trials. Studies were undertaken in mice to develop a suitable formulation and analytical methodology for use in pharmacokinetic studies and to define the plasma pharmacokinetics, tissue distribution, and urinary excretion of Pc4 after i.v. delivery. Methods: An HPLC method suitable for separation and quantification of Pc4 was developed and validated for use in mouse plasma, tissues, and urine. The stability of Pc4 was characterized in a variety of formulations as well as in mouse plasma. Before pursuing pharmacokinetic studies, preliminary toxicity studies were undertaken. These studies utilized Pc4 formulated in diluent 12:0.154 M NaCl (1:3, v:v). Pharmacokinetic studies involved Pc4 doses of 40 mg/kg, 10 mg/kg and 2 mg/kg administered as i.v. boluses to female, CD2F1 mice . Doses of 40 mg/kg, 10 mg/kg, and 2 mg/kg were studied with drug formulated in diluent 12:0.154 M NaCl (1:3, v:v). Doses of 10 mg/kg and 2 mg/kg were also studied with drug formulated in a vehicle consisting of polyethylene glycol:Tween 80:0.01 M sodium phosphate buffer, pH 7.0 (40:0.2:59.8, v:v:v). Compartmental and non-compartmental analyses were applied to the plasma concentration-versus-time data. Concentrations of Pc4 were also determined in a variety of tissues, including brain, lung, liver, kidney, skeletal muscle, skin, heart, spleen, and abdominal fat. Urine was collected from animals treated with each of the doses of Pc4 mentioned above, and daily, as well as cumulative drug excretion was calculated until 168 h after treatment. Results: At a dose of 80 mg/kg, two of five male and two of five female mice were dead by 24 h after injection. Pathologic examination revealed gross findings of blue discoloration affecting many tissues, with lungs that were grossly hemorrhagic and very blue-black. Microscopic examination of the lungs revealed mild acute interstitial pneumonia, with perivascular edema and inflammation, and a detectable margination of neutrophils around larger pulmonary blood vessels. Animals sacrificed 14 days after treatment showed mild granulomatous pneumonia, characterized by clusters of multi-nucleated giant cells, with fewer macrophages and neutrophils. The giant cells frequently contained phagocytized particles, which were clear and relatively fusiform. All mice treated with 40 mg/kg or 20 mg/kg survived and returned to pretreatment weight during the 14 days after treatment. Intravenous bolus delivery of Pc4, at a dose of 40 mg/kg, produced “peak” plasma Pc4 concentrations between 7.81 and 8.92 μg/ml in mice killed at 5 min after injection (the earliest time studied after drug delivery). Sequential reduction of the Pc4 dose to 10 mg/kg in diluent 12:0.154 M NaCl (1:3, v:v), 10 mg/kg in polyethylene glycol:Tween 80:sodium phosphate buffer (40:0.2:59.8, v:v:v), 2 mg/kg in diluent 12:0.154 M NaCl (1:3, v:v), and, finally, 2 mg/kg in polyethylene glycol:Tween 80:sodium phosphate buffer (40:0.2:59.8, v:v:v) resulted in “peak” plasma Pc4 concentrations between 2.07 and 3.24, 0.68 and 0.98 μg/ml, and 0.29 and 0.41 μg/ml, respectively. Pc4 persisted in plasma for prolonged periods of time (72–168 h). Non-compartmental analysis of plasma Pc4 concentration-versus-time data showed an increase in area under the plasma Pc4 concentration-versus-time curve (AUC) when the dose of Pc4 increased from 2 mg/kg to 40 mg/kg. Across the 20-fold range of doses studied, total body clearance (CLtb) varied from 376 to 1106 ml h−1 kg−1. Compartmental modeling of plasma Pc4 concentration versus time data showed the data to be fit best by a two-compartment, open, linear model. Minimal amounts of Pc4 were detected in the urine of mice. After i.v. bolus delivery to mice, Pc4 distributed rapidly to all tissues and persisted in most tissues for the duration of each pharmacokinetic study. Tissue exposure, as measured by AUC, increased in a dose-dependent fashion. Conclusions: The HPLC method developed for quantification of Pc4 in plasma, urine, and tissues should be suitable for clinical studies of the drug. Pc4 is widely distributed and persists in plasma and tissues of mice for prolonged periods of time. These data are relevant to the design of forthcoming clinical trials of Pc4.