Carl W. Gilbert

Camptothecin delivery systems : enhanced efficacy and tumor accumulation of camptothecin following its conjugation to polyethylene glycol via a glycine linker

Purpose: This study was designed to assess the circulatory retention, antitumor activity and tissue biodistribution of polyethylene glycol (PEG)-conjugated camptothecin-20-O-glycinate, PEG-β-camptothecin (PEG-β-CPT). PEG-β-CPT is a novel water-soluble transport form (macromolecular prodrug) of the naturally derived antitumor drug, 20-(S )-camptothecin (CPT). Methods: Circulatory retention studies were performed in nontumor-bearing mice injected intravenously (i.v.) with 875 mg/kg of PEG-β-CPT. Antitumor activity was evaluated both intraperitoneally (i.p.) and i.v. in nude mouse xenograft models. Biodistribution studies were performed in nude mice bearing colorectal carcinoma xenografts with tritium-labelled PEG-β-CPT and CPT injected i.v. Results: PEG-β-CPT had a blood t1/2α of approximately 6 min and a t1/2β of 10.2 h. Significant antitumor activity was seen in all treated xenograft models. Biodistribution studies demonstrated that PEG-β-CPT in saline provided more available labelled CPT in the circulation than unconjugated CPT dissolved in intralipid. In addition, it appeared that more labelled CPT accumulated in solid tumors when delivered in the PEG-β-CPT form, with greater preference for tumor tissue than normal tissue. Conclusion: This soluble transport form of CPT and its underlying technology may have clinical application especially for the treatment of solid tumors.

Camptothecin-20-PEG ester transport forms: the effect of spacer groups on antitumor activity.

An improved synthesis of the hindered PEG-camptothecin diester transport form has been achieved using the Mukaiyama reagent. We have also assessed the effect of changing the electronic configuration of the (d-position of PEG-camptothecin transport forms on the rates of hydrolysis of the pro-moiety, and attempted to correlate these differences to efficacy in two animal models. In addition to the simple substitution of N for O, other synthetic modifications of these atoms were accomplished by employing heterobifunctional linker groups. The half lives by disappearance (rates of hydrolysis) of the transport forms in buffer and rat plasma were determined. It was established that anchimeric assistance to hydrolytic breakdown of the pro-moiety occurs in a predictable manner for some of these compounds. Results for the new derivatives in a P388 murine leukemic model and HT-29 human colorectal xenograft study are also presented. The use of a glycine linker group was found to provide similar efficacy in rodent models to that of simple camptothecin 20-PEG ester, and displayed enhanced pharmacokinetics.

Highly water soluble taxol derivatives: 2′-polyethyleneglycol esters as potential prodrugs

Abstract 2′ and 7-polyethylene glycol esters of taxol were prepared and found to be essentially water soluble. The rates of hydrolysis of these compounds were measured under neutral, acidic and physiological (basic) conditions. The half-lives of O and N substituted 2′-esters are short enough to permit their use as produgs.

Peg-Hemoglobin:an Efficient Oxygen-Delivery System in the Rat Exchange Transfusion and Hypovolemic Shock Models

Polyethylene glycol-hemoglobin (PEG-Hb is a purified bovine hemoglobin molecule modified by polyethylene glycol. Oxygen delivery to the tissue, in rat exchange transfusion and hypovolemic shock models, was studied to determine whether the oxygen-carrying capacity of PEG-Hb is as efficient as red blood cells.

PEG Modified Anticancer Drugs: Synthesis and Biological Activity:

Three well known anticancer drugs, doxorubicin, methotrexate and paclitaxel (taxol), have been modified by the permanent attachment of poly(ethylene glycol)(PEG) and evaluated for in vitro cytotoxic activity against murine leukemias P388 and L1210. The relative potencies of the PEG derivatives suggest that modification of antitumor agents with this type of polymer yields compounds that are highly water soluble but less cytotoxic.

Changes in the functional properties of bovine hemoglobin induced by covalent modification with polyethylene glycol.

Polyethylene glycol conjugation to proteins and peptides (PEGylation) has been shown to promote increased retention time in the circulation as well as to blunt immune or allergic reactions. PEGylated bovine hemoglobin (PEG-Hb) is being explored in human clinical trials as an oxygen delivering agent for the sensitization of solid tumors to radiation therapy. In this study the functional properties of PEG-Hb were compared to those of bovine hemoglobin (Hb), the mutant human hemoglobin Rothchild and bovine hemoglobin crosslinked between the beta chains. The rate of heme transfer from Hb to serum albumin at pH 9.0 was greatly increased by PEGylation, suggesting destabilization of the heme-globin linkage and of the bonds between alpha beta dimers. Measurement of oxygen binding equilibrium showed that the oxygen affinity of Hb became unusually dependent on temperature and Hb concentration after PEGylation. Evidence is presented to suggest that PEGylation of lysine beta-81 at the entrance to the central cavity of the Hb tetramer might be responsible for these observations. The alterations of the functional properties of Hb induced by PEGylation are consistent with the beneficial effects of PEG-Hb in exchange transfusion and radiation sensitization models of human conditions.

EVALUATION OF THE OXYGEN DELIVERY ABILITY OF PEG- HEMOGLOBIN IN SPRAGUE-DAWLEY RATS DURING HEMODILUTION

Polyethylene glycol (PEG) conjugation allows bovine hemoglobin (Hb) to retain its oxygen delivery capability while increasing its plasma expansion capacity. To determine whether PEG-Hbs ability to sustain life is due to its oxygen delivery capability rather than its plasma expansion capacity, Sprague-Dawley rats were exchange-transfused up to an 85% hematocrit reduction with either PEG-Hb, PEG-50%-methemoglobin (PEG-mHb), PEG-carbon monoxide hemoglobin (PEG-COHb) or PEG-human serum albumin (PEG-HSA). Survival and respiratory rates were monitored during the exchange transfusion, at five minutes, 24 hours and 48 hours post operative. Rats surviving 14 days were evaluated for hematology, blood chemistry and histopathology. Rats infused with PEG-Hb had a survival rate of 100% during the transfusion and 79% at 24 hours, as compared to 24 hour survival rates of 30% for PEG-mHb, and 0% for both PEG-COHb and PEG-HSA. PEG-Hb treated rats that survived the 2 week observation period had normal hematological and blood chemistry levels and no significant morphological effects. Therefore, this study demonstrates that PEG-Hb can sustain life while similar plasma expansion agents with less oxygen delivery capability are not as effective.

Normal oxygen tension restored in the ischemic rat liver model by PEG-hemoglobin.

Cell damage initiated during ischemia, as a result of oxygen depletion, continues during reperfusion, and recovery is dependent on the length of the ischemic period. This study investigates the effect of polyethylene glycol-modified hemoglobin (PEG-Hb) on recovery of tissue oxygen tension after induced ischemia.