Herein, new biodegradable star
polymer-
doxorubicin conjugates designed for passive
tumor targeting were investigated, and their synthesis, physico-chemical characterization, drug release, biodegradation, biodistribution and in vivo anti-
tumor efficacy are described. In the conjugates, the core formed by
poly(amidoamine) (PAMAM)
dendrimers was grafted with semitelechelic
N-(2-hydroxypropyl)methacrylamide (
HPMA) copolymers bearing
doxorubicin (Dox) attached by
hydrazone bonds, which enabled intracellular pH-controlled drug release. The described synthesis facilitated the preparation of biodegradable
polymer conjugates in a broad range of molecular weights (200-1000g/mol) while still maintaining low polydispersity (~1.7). The
polymer grafts were attached to the
dendrimers through either stable
amide bonds or enzymatically or reductively degradable spacers, which enabled intracellular degradation of the high-molecular-weight
polymer carrier to excretable products. Biodegradability tests in
suspensions of EL4
T-cell lymphoma cells showed that the rate of degradation was much faster for reductively degradable conjugates (close to completion within 24h of incubation) than for conjugates linked via an enzymatically degradable
oligopeptide GFLG sequence (slow degradation taking several days). This finding was likely due to the differences in steric hindrance in terms of the accessibility of the small molecule
glutathione and the bulky
enzyme cathepsin B to the
polymer substrate. Regarding drug release, the conjugates were fairly stable in
buffer at pH 7.4 (model of blood stream) but released
doxorubicin under mild acidic conditions that model the
tumor cell microenvironment. The star
polymer-Dox conjugates exhibited significantly prolonged blood circulation and enhanced
tumor accumulation in
tumor-bearing mice, indicating the important role of the EPR effect in its anti-
cancer activity. The star
polymer conjugates showed prominently higher in vivo anti-
tumor activities than the free
drug or linear
polymer conjugate when tested in mice bearing EL4
T-cell lymphoma, with a significant number of long-term surviving (LTS). Based on the results, we conclude that a M(w) of
HPMA copolymers of 200,000 to 600,000g/mol is optimal for
polymer carriers designed for the efficient passive targeting to solid
tumors. In addition, an expressive
therapy-dependent stimulation of the immune system was observed.