Characterization of Polyelectrolyte Complex Formation Between Anionic and Cationic Poly(amino acids) and Their Potential Applications in pH-Dependent Drug Delivery

Polyelectrolyte complexes (PECs) are self-assembling nano-sized constructs that offer several advantages over traditional nanoparticle carriers including controllable size, biodegradability, biocompatibility, and lack of toxicity, making them particularly appealing as tools for drug delivery. Here, we discuss potential application of PECs for drug delivery to the slightly acidic tumor microenvironment, a pH in the range of 6.5–7.0. Poly(l-glutamic acid) (E(n)), poly(l-lysine) (K(n)), and a copolymer composed of histidine-glutamic acid repeats ((HE)(n)) were studied for their ability to form PECs, which were analyzed for size, polydispersity, and pH sensitivity. PECs showed concentration dependent size variation at residue lengths of E(51)/K(55) and E(135)/K(127), however, no complexes were observed when E(22) or K(21) were used, even in combination with the longer chains. (HE)(20)/K(55) PECs could encapsulate daunomycin, were stable from pH 7.4–6.5, and dissociated completely between pH 6.5–6.0. Conversely, the E(51-dauno)/K(55) PEC dissociated between pH 4.0 and 3.0. These values for pH-dependent particle dissociation are consistent with the pK(a)’s of the ionizable groups in each formulation and indicate that the specific pH-sensitivity of (HE)(20-dauno)/K(55) PECs is mediated by incorporation of histidine. This response within a pH range that is physiologically relevant to the acidic tumors suggests a potential application of these PECs in pH-dependent drug delivery.