Design and development of high-drug-load paclitaxel (PTX)-antibody conjugates: Synthesis and preliminary evaluation of soluble paclitaxel (sPTX) and sPTX-cetuximab conjugates

Abstract

e14634 Background: Tumor cell surface receptor-targeted therapy of cancer is an effective treatment strategy and monoclonal antibody (MAb)-drug conjugates hold promise as tumor-specific drug delivery systems. We have previously reported the synthesis and biological evaluation of paclitaxel (PTX)-MAb conjugates using the anti-EGFR MAb, cetuximab, as an agent for tumor-targeted drug delivery system (Safavy et al., Bioconjugate Chem 14:302–310, 2003). One limitation of these conjugates was a low drug load, i.e., PTX : MAb ratios of ≤4, and thereby, low target dose delivery. Attempts to synthesize conjugates with PTX : MAb of >4 failed due to their low water solubility. As the source of this problem was identified to be the highly lipophilic PTX moiety, we hypothesized that conjugation of water-soluble PTX analogues may lead to the preservation of the MAb solubility, and thus, to the preparation of conjugates with high PTX : MAb ratios. Methods: Newly available “discrete” poly(ethylene glycol) (dPEG) linkers were utilized for the synthesis of soluble PTX analogues. In contrast to the regular PEGs wit dispersed molecular weight ranges (MWs), the dPEGs are single-MW compounds which make possible the exact dose calculation for the resulting conjugates. Solid-phase peptide synthesis methodology was used to prepare a trifunctional dPEG-lysine (K) linker, which was first conjugated to PTX to yield the water-soluble drug analogue sPTXdPEGKdPEG, containing a free carboxylic acid group. Using the latter functionality, sPTXdPEGLysdPEG was covalently conjugated to cetuximab (C225) at increasing molar ratios. Cell binding assays were performed using MCF-7 human breast carcinoma cell line. Results: sPTXC225 conjugates with PTX : C225 ratios of up to 8 were synthesized through this method, which in contrast to the previously reported conjugates, demonstrated complete aqueous solubility. Cell binding assays indicated preservation of the immunoreactivity of the parent MAb in all conjugates. Conclusions: Water-soluble and high-drug-load PTX-MAbs such as sPTXC225, may be useful for targeted delivery of therapeutically effective doses of paclitaxel and warrant further investigation. No significant financial relationships to disclose.