HPMA-based star polymer biomaterials with tuneable structure and biodegradability tailored for advanced drug delivery to solid tumours

Abstract

Design, controlled synthesis, physico-chemical and biological characteristics of novel well-defined biodegradable star-shaped copolymers intended for advanced drug delivery is described. These new biocompatible star copolymers were synthesised by grafting monodispersed semitelechelic linear (sL) N-(2-hydroxypropyl)methacrylamide copolymers onto a 2,2-bis(hydroxymethyl)propionic acid (bisMPA)-based polyester dendritic core of various structures. The hydrodynamic diameter of the star copolymer biomaterials can be tuned from 13 to 31 nm and could be adjusted to a given purpose by proper selection of the bisMPA dendritic core type and generation and by considering the sL copolymer molecular weight and polymer-to-core molar ratio. The hydrolytic degradation was proved for both the star copolymers containing either dendron or dendrimer core, showing the spontaneous hydrolysis in duration of few weeks. Finally, it was shown that the therapy with the biodegradable star conjugate with attached doxorubicin strongly suppresses the tumour growth in mice and is fully curative in most of the treated animals at dose corresponding approximately to one fourth of maximum tolerated dose (MTD) value. Both new biodegradable systems show superior efficacy and tumour accumulation over the first generation of star copolymers containing non-degradable PAMAM core.

Keywords: Cancer; Doxorubicin; Drug delivery; HPMA; Star-like polymers; bisMPA.

Figure 1.

Reaction scheme of RAFT polymerisation of linear precursors followed by TTc end group removal.

Figure 2.

Reaction scheme of star-like polymer formulation. Left: chemical structures of dendron (up) or dendrimer (down). Right: structure of the HPMA-based copolymer. The reaction steps leading to the drug conjugate are shown in the lower frame.

Figure 3.

Kinetics of the formation of star-like polymer precursors. ■ : % of remaining TT groups on the polymer chains; ♦ : % of unreacted linear polymer; ⚫ : molar mass of star-like polymer.

Figure 4.

Degradation study of star-like polymer S1 with azido groups on dendrimer core (A) and star-like polymer S2 with amino groups on dendron core (B). Percent of linear polymer was estimated from SEC from the RI detector as the ratio between the area under peaks corresponding to star-like and linear polymer. ♦ pH 7.4 ◊ pH 6.5 ♦ pH 5.0

Figure 5.

Whole-body retention of radioactivity, upon intravenous injection of star-like conjugates into 4T1 tumour-bearing mice (n = 4); ■ S3-DFO; ▲ S2-DFO.

Figure 6.

Overall survival and tumor growth of EL-4 lymphoma-bearing mice treated with polymer conjugates C1-C4. C57BL/6 mice were transplanted s.c. with 1× 10⁵ EL4 cells and treated with 7.5 mg DOX eq./kg of conjugates C1-C4 (A) or with a single dose of 7.5 mg DOX eq./kg or two doses of 5 mg DOX eq./kg of conjugates C1 and C2 (B). Tumour growth and survival were monitored. The figure summarises the results of two independent experiments with eight mice in each experimental group. The difference between the untreated and all treated groups (A-I, B-I) was highly significant (p<0.0001). The difference between the C1, C2 and C3 treated groups (A-I) was not significant. The difference between well-defined conjugates C1-C3 and C4 was significant (p<0.001). Arrows indicate the dosing points. Black arrows indicate the injection of the single dose, white arrows indicate the injections in the two-dosing scheme. Figure 6 - C and D: Survival of mice with fully regressed EL4 T cell lymphomas following treatment with bisMPA-based DOX conjugates (as described in Fig. 6A, 6B). At 170 days after first tumour transplantation, mice were injected s.c. with 1×10⁵ EL4 cells and left untreated. Control = naive mice injected with EL4 cells. Tumour growth (not shown) and survival were monitored.

Figure 7.

DOX accumulation in tumour (A) and liver (B) after administration of star polymer-DOX biomaterials. Dark bars: bisMPA-dendron based C2; light bars: bisMPA-dendrimer based C1; grey striped bars: PAMAM-based C3.

Figure 8.

Liver (8A) and tumour (8B) retention of radioactivity upon intravenous injection of star-like conjugates into 4T1 tumour-bearing mice (n = 4); ■ S2-DFO; ▼ S3-DFO.