Drug-loaded, bivalent-bottle-brush polymers by graft-through ROMP

Abstract

Graft-through ring-opening metathesis polymerization (ROMP) using ruthenium N-heterocyclic carbene catalysts has enabled the synthesis of bottle-brush polymers with unprecedented ease and control. Here we report the first bivalent-brush polymers; these materials were prepared by graft-through ROMP of drug-loaded polyethylene-glycol (PEG) based macromonomers (MMs). Anticancer drugs doxorubicin (DOX) and camptothecin (CT) were attached to a norbornene-alkyne-PEG MM via a photocleavable linker. ROMP of either or both drug-loaded MMs generated brush homo- and co-polymers with low polydispersities and defined molecular weights. Release of free DOX and CT from these materials was initiated by exposure to 365 nm light. All of the CT and DOX polymers were at least 10-fold more toxic to human cancer cells after photoinitiated drug release while a copolymer carrying both CT and DOX displayed 30-fold increased toxicity upon irradiation. Graft-through ROMP of drug-loaded macromonomers provides a general method for the systematic study of structure-function relationships for stimuli-responsive polymers in biological systems.

Figure 1

Schematic depiction of bivalent macromonomer (MM) and bivalent-brush polymer described in this work.

Figure 2

MALDI spectra for PEG-norbornene 3 before (black trace) and after (red trace) CuAAC click coupling of CT-NBOC-N₃ to give CT-MM. The observed mass shift of 653 Da agrees with the calculated mass of CT-NBOC-N₃ and confirms successful attachment of the photocleavable drug moiety.

Figure 3

Representative GPC traces for brush polymer samples. Black and red chromatograms correspond to crude ROMP reaction mixtures. The brush polymers display narrowly dispersed, mono-modal molecular weight distributions. The GPC trace for purified pDOX02 is shown in orange, indicating that it is possible to remove trace MM impurity.

Figure 4

HPLC-MS traces of aqueous brush polymer solutions before and after 365 nm UV irradiation for various times. pDOX02 and pCT03 yield free DOX and CT, respectively. LC-MS method A (see methods and materials) was used for pDOX02 while method B was used for pCT03. Inset mass spectra, obtained from the free DOX and CT peaks, show strong signals at m:z ratios that correspond to the molecular ions of DOX and CT.

Figure 5

A: UV-Vis absorption of pDOX01, pCT01, and copolymer pDOX₅₀-pCT₅₀. B: HPLC traces of an aqueous solution of pDOX₅₀-pCT₅₀ before and after 365 nm UV irradiation for 10 min.

Figure 6

A–C: Viability of MCF-7 human breast cancer cells treated with free DOX and CT and drug-loaded brush polymers both with and without UV irradiation. Data points were fit to a sigmoidal function and the half-maximum inhibitory concentrations (IC₅₀) are shown. The x-axis labels refer to the concentration of both free and polymer-conjugated drug. D: Table of therapeutic factors for each brush polymer formulation. These values represent the fold-increase in toxicity after irradiation and drug release.

Scheme 1

Synthesis of PEG-norbornene-alkyne macromonomer 3.

Scheme 2

Synthesis of clickable, photocleavable drugs CT-NBOC-N₃ and DOX-NBOC-N₃.

Scheme 3

Click coupling of 3 to photocleavable drug derivatives.

Scheme 4

Synthesis and structure of poly(norbornene)-PEG brush polymers with DOX or CT attached via a photocleavable NBOC linker.