. To this end, we generated a library of >500 degradable, poly(␤-amino esters) for potential use as nonviral DNA vectors. Using high-throughput methods, we screened this library in vitro for transfection efficiency and cytotoxicity. We tested the best performing polymer, C32, in mice for toxicity and DNA delivery after intratumor and i.m. injection. C32 delivered DNA intratumorally Ϸ4-fold better than one of the best commercially available reagents, jetPEI (polyethyleneimine), and 26-fold better than naked DNA. Conversely, the highest transfection levels after i.m. administration were achieved with naked DNA, followed by polyethyleneimine; transfection was rarely observed with C32. Additionally, polyethyleneimine induced significant local toxicity after i.m. injection, whereas C32 demonstrated no toxicity. Finally, we used C32 to deliver a DNA construct encoding the A chain of diphtheria toxin (DT-A) to xenografts derived from LNCaP human prostate cancer cells. This construct regulates toxin expression both at the transcriptional level by the use of a chimeric-modified enhancer͞promoter sequence of the human prostate-specific antigen gene and by DNA recombination mediated by Flp recombinase. C32 delivery of the A chain of diphtheria toxin DNA to LNCaP xenografts suppressed tumor growth and even caused 40% of tumors to regress in size. Because C32 transfects tumors locally at high levels, transfects healthy muscle poorly, and displays no toxicity, it may provide a vehicle for the local treatment of cancer.prostate ͉ cationic polymers

Poly(beta-amino ester)s are cationic degradable polymers that have significant potential as gene delivery vectors. Here we present a generalized method to modify poly(beta-amino ester)s at the chain ends to improve their delivery performance. End-chain coupling reactions were developed so that polymers could be synthesized and tested in a high-throughput manner, without the need for purification. In this way, many structural variations at the polymer terminus could be rapidly evaluated. End-modification of the terminal amine structure of a previously optimized poly(beta-amino ester), C32, significantly enhanced its in vitro transfection efficiency. In vivo, intraperitoneal (IP) gene delivery using end-modified C32 polymers resulted in expression levels over one order of magnitude higher than unmodified C32 and jet-polyethylenimine (jet-PEI) levels in several abdominal organs. The rapid end-modification strategy presented here has led to the discovery of many effective polymers for gene delivery and may be a useful method to develop and optimize cationic polymers for gene therapy.