The effect of polymer backbone chemistry on the induction of the accelerated blood clearance in polymer modified liposomes

Abstract

A variety of water-soluble polymers, when attached to a liposome, substantially increase liposome circulation half-life in animals. However, in certain conditions, liposomes modified with the most widely used polymer, polyethylene glycol (PEG), induce an IgM response resulting in an accelerated blood clearance (ABC) of the liposome upon the second injection. Modification of liposomes with other water-soluble polymers: HPMA (poly[N-(2-hydroxypropyl) methacrylamide]), PVP (poly(vinylpyrrolidone)), PMOX (poly(2-methyl-2-oxazoline)), PDMA (poly(N,N-dimethyl acrylamide)), and PAcM (poly(N-acryloyl morpholine)), increases circulation times of liposomes; but a precise comparison of their ability to promote long circulation or induce the ABC effect has not been reported. To obtain a more nuanced understanding of the role of polymer structure/MW to promote long circulation, we synthesized a library of polymer diacyl chain lipids with low polydispersity (1.04-1.09), similar polymer molecular weights (2.1-2.5kDa) and incorporated them into 100nm liposomes of a narrow polydispersity (0.25-1.3) composed of polymer-lipid/hydrogenated soy phosphatidylcholine/cholesterol/diD: 5.0/54.5/40/0.5. We confirm that HPMA, PVP, PMOX, PDMA and PAcM modified liposome have increased circulation times in rodents and that PVP, PDMA, and PAcM do not induce the ABC effect. We demonstrate for the first time, that HPMA does not cause an ABC effect whereas PMOX induces a pronounced ABC effect in rats. We find that a single dose of liposomes coated with PEG and PMOX generates an IgM response in rats towards the respective polymer. Finally, in this homologous polymer series, we observe a positive correlation (R=0.84 in rats, R=0.92 in mice) between the circulation time of polymer-modified liposomes and polymer viscosity; PEG and PMOX, the polymers that can initiate an ABC response were the two most viscous polymers. Our findings suggest that polymers that do not cause an ABC effect such as, HPMA or PVP, deserve further consideration as polymer coatings to improve the circulation of liposomes and other nanoparticles.

Keywords: Accelerated blood clearance; Anti-polymer IgM; Biodistribution; Pharmacokinetics; Polyethylene glycol; Polymer modified liposomes; RAFT polymerization.

Figure 1

Elimination of a single dose of liposomes from blood in rats over 48 h. (Mean +/− SEM, n=3)

Figure 2

Elimination of the second dose of liposomes from blood in rats administered one week after the first dose measured over 48 h after administration of the second dose. (Mean +/− SEM, n=3–4)

Figure 3

Biodistribution of the first and second dose of liposomes in rats 48 h after dosing. (n = 3 per group for animals receiving a single dose, n = 4 per group for animals receiving 2 doses). Only the PEG and PMOX modified liposomes had significantly different liposome accumulation in the liver and spleen over the two doses (^**, p < 0.01).

Figure 5

Viscosity of 10 kDa polymers in aqueous conditions vs the circulation half-life of liposomes incorporating 2 kDa polymers in rats (circles) and mice (squares). Red, PEG; dark blue,PMOX; irish green, HPMA; light blue, PVP; purple, PDMA; dark green, PAcM.

Scheme 1

Synthetic schemes for HPMA polymerization, PAcM polymerization, PDMA polymerization, PVP polymerization (A, top to bottom), general polymer conjugation to MCC PE lipid (B), PMOX polymerization followed by lipid conjugation (C) (D).