Optimization of intraperitoneal topotecan chemotherapy with anti-topotecan antibodies
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This dissertation presents an approach to optimize topotecan (TPT) intraperitoneal chemotherapy using anti-topotecan antibodies (ATAb). We hypothesize that ATAb may be used to decrease systemic unbound TPT exposure (AUC f ), decrease TPT-induced systemic toxicity and enhance the efficacy of TPT intraperitoneal chemotherapy. The limitations of 'traditional' targeting strategies, as well as different 'inverse targeting strategies' to improve the regio-selectivity of drug disposition were reviewed. A sensitive HPLC assay was developed to quantify total and unbound TPT plasma concentrations in mice, with or without the presence of anti-TPT antibodies. The assay has a working range of 1-500 ng/ml, with lower limit of quantitation of 0.02 ng. Pharmacokinetic studies suggested that TPT demonstrated "non-restrictive" elimination, as the clearance was relatively insensitive to changes in protein binding. TPT pharmacokinetics/toxicodynamics were assessed in mice following a wide range of dose and dosing protocols (i.v. bolus, i.p. bolus and i.p. infusion from 24-168 h). An integrated PK/PD model was developed to describe the relationship between TPT exposure and TPT-induced toxicity in mice. A hybridoma cell line secreting high affinity anti-TPT immunoglubin (K A = 4.8 × 10 8 M -1 ) was developed. Anti-TPT IgG and Fab fragments were produced, purified, and characterized. The effects of endogenous ATAb on TPT pharmacokinetics/toxicodynamics were investigated in mice that had been immunized with a TPT-immunoconjugate, and the therapeutic efficacy of TPT i.p. chemotherapy was evaluated in mice bearing peritoneal tumors. Results showed that immunized animals demonstrated a dramatic decrease in AUC f relative to the non-immunized animals. Endogenous ATAb protected immunized animals from severe body weight loss and allowed a 3-fold increase of the maximum tolerated dose of i.p. TPT. Additionally, immunized mice bearing peritoneal tumors showed improved survival relative to non-immunized tumor-bearing mice (100% vs. 20%). Finally, a hybrid compartmental/physiological pharmacokinetic model was developed to describe the effect of endogenous ATAb on TPT disposition. Simulations using this model predicted that the dissociation rate constant (k off ) may be the most important determinant of antibody effect, and an antibody with intermediate binding affinity (KA [approximate] 1 × 10 7 M -1 ) may be the most efficient to decrease systemic unbound drug exposure following intraperitoneal chemotherapy.