6(A),6(D)-Bis-(2-amino-2-carboxylethylthio)-6(A),6(D)-dideoxy-beta-cyclodextrin 1, a diamino acid derivative of beta-cyclodextrin, is synthesized and condensed with difunctionalized PEG comonomers to give linear, high molecular weight (M(w) over 50 kDa) beta-cyclodextrin-based polymers (2-4) with pendant functionality (carboxylate). 2-4 are all highly soluble in aqueous solutions (over 200 mg/mL). 20-O-trifluoroglycinylcamptothecin, 5a, and 20-O-trifluoroglycinylglycinylglycinylcamptothecin, 5b, are synthesized and conjugated to 2 to give polymer-camptothecin (CPT) prodrugs. The solubility of CPT is increased by more than three orders of magnitude when it is conjugated to 2. The rates of CPT release from the conjugates HGGG6 (high molecular weight polymer (M(w) 97 kDa), glyglygly linker and 6 wt % CPT loading) and HG6 (high MW polymer (M(w) 97 kDa), gly linker and 6 wt % CPT loading) in either mouse or human plasma are dramatically accelerated over the rates of pure hydrolysis at pH = 7.4, indicating the presence of enzymatic cleavage as a rate-determining step at this pH in the release of the CPT. The pH of aqueous solution has a large effect on hydrolysis rate of CPT from HGGG6 and HG6; the lower the pH, the slower the rate in the range at 4.1
The studies presented here indicate that intravenous administration of IT-101, a cyclodextrin based polymer-CPT conjugate, gives prolonged plasma half-life and enhanced distribution to tumor tissue when compared to CPT alone. The data also show that active CPT is released from the conjugate within the tumor for an extended period of time. These effects likely play a significant role in the enhanced antitumor activity of IT-101 when compared to CPT alone or irinotecan.
Antitumor activity of linear, beta-cyclodextrin polymer (CDP)-camptothecin (CPT) conjugates (HGGG6, LGGG10, HG6, and HGGG10) is investigated in nude mice bearing human LS174T colon carcinoma tumors. These conjugates differ in polymer molecular mass [97 kDa (H) or 35 kDa (L)], CDP-CPT linker structure [glycine (G) or triglycine (GGG)], and CPT loading [ca. 6 wt % (6) or 10 wt % (10)]. Maximum tolerable doses (MTDs) of the three conjugates, LGGG10, HG6, and HGGG10, are determined to be 36, 9, and 9 mg of CPT/kg, respectively, while the MTD of the CDP alone exceeds 240 mg/kg (highest value investigated). The three CDP-CPT conjugates with high polymer molecular masses (HGGG6, HG6, and HGGG10) demonstrate antitumor activity at their MTDs superior to that of CPT at the same amount and to that of irinotecan at its optimal dose. They also show tumor growth inhibition that is superior to that of the conjugate containing the low-molecular mass polymer (LGGG10) at the same dose of CPT. No significant effects of CPT weight loading or linker structure on tumor growth delay are observed. However, conjugates containing G appear to be less toxic than these with GGG. These antitumor studies demonstrate that the CDP-based conjugates of CPT exhibit tumor growth inhibition superior to that of CPT or irinotecan at the conditions employed in this study. The striking observation is that a short course of treatment with the polymer conjugates gives long-term control of tumor growth that does not occur with either CPT or irinotecan. Intracellular CDPs are demonstrated by analyzing cells that were cultured in the presence of rhodamine-labeled CDP (HRhod) containing medium using both confocal microscopy and flow cytometry. The long-term therapeutic efficacy of CDP-CPT conjugates observed in mice may in part be due to the sustained release of CPT from these conjugates in the acidic, intracellular compartments since these conjugates are shown to have significantly slower release rates at acidic pH than at physiological pH.
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