Synthesis and Endosomolytic Properties of Poly(amidoamine) Block Copolymers

The poly(amidoamine)s (PAAs) ISA 1 and ISA 23 display pH‐dependent conformational change and pH‐dependent membrane perturbation. These properties confer potential for use as endosomolytic polymers for intracytoplasmic delivery of toxins and genes. Both polymers are relatively non‐toxic, and moreover ISA 23 has the beneficial property in vivo, of being non hepatotropic when administered intravenously. Although ISA 23 and ISA 1 demonstrate ability to transfect cells, ISA 1 is also able to promote intracellular delivery of non‐permeant toxins. The aim of this study was to synthesise random and block copolymers of ISA 1 and ISA 23 and investigate whether these second generation hybrids would allow optimisation of PAA biological characteristics. Random and block copolymers of ISA 1 and ISA 23 were synthesised by hydrogen transfer polyaddition to generate a library of PAAs with an ISA 23:ISA 1 molar ratios of 2:1 to 4:1. The resultant polymers have a pI slightly below 7.4 and a $\overline M _{\rm w}$ of 19 900–49 000 g/mol and a $\overline M _{\rm n}$ of 13 100–24 100 g/mol. Whereas none of the random or block copolymers were haemolytic at pH 7.4 all demonstrated pH‐dependent membrane activity. At pH 5.5 they caused 50–60% haemoglobin (Hb) release over 1 h. This was slightly less than that seen for ISA 23 (80% Hb release). None of the copolymers were cytotoxic against B16F10 cells during a 72 h incubation (IC50 > 2 mg/ml; MTT assay). The ability of the random and block copolymer PAAs to deliver the toxin gelonin was also examined, but only ISA 1 and the block copolymer B2 (ISA 23:ISA 1 at a 2:1 molar ratio) were able to promote intracellular delivery, as measured by cytotoxic activity. It would be interesting to study the body distribution of B2 and determine whether this toxin‐delivering PAA is able to escape liver capture.