Nanoparticulate formulations of mithramycin analogs for enhanced cytotoxicity

Abstract

Mithramycin (MTM), a natural product of soil bacteria from the Streptomyces genus, displays potent anticancer activity but has been limited clinically by severe side effects and toxicities. Engineering of the MTM biosynthetic pathway has produced the 3-side-chain-modified analogs MTM SK (SK) and MTM SDK (SDK), which have exhibited increased anticancer activity and improved therapeutic index. However, these analogs still suffer from low bioavailability, short plasma retention time, and low tumor accumulation. In an effort to aid with these shortcomings, two nanoparticulate formulations, poly(ethylene glycol)-poly(aspartate hydrazide) self-assembled and cross-linked micelles, were investigated with regard to the ability to load and pH dependently release the drugs. Micelles were successfully formed with both nanoparticulate formulations of each drug analog, with an average size of 8.36 ± 3.21 and 12.19 ± 2.77 nm for the SK and SDK micelles and 29.56 ± 4.67 nm and 30.48 ± 7.00 nm for the SK and SDK cross-linked micelles respectively. All of the drug-loaded formulations showed a pH-dependent release of the drugs, which was accelerated as pH decreased from 7.4 to 5.0. The micelles retained biological activity of SK and SDK entrapped in the micelles, suppressing human A549 lung cancer cells effectively.

Keywords: MTM; cancer chemotherapy; controlled release; drug delivery; polymer micelles.

Figure 1

Self-assembled and cross-linked polymer micelles as nanoparticulate formulations for MTM analogs. Abbreviation: MTM, mithramycin.

Figure 2

Synthesis of block copolymers, cross-linked block copolymers, and drug conjugates.

Figure 3

(A) Particle size and (B) drug-entrapment yields. Abbreviations: SDK, mithramycin SDK; SK, mithramycin SK.

Figure 4

Drug-release patterns from self-assembled and cross-linked micelles at pH 7.4 and 5.0 for 72 hours. Notes: All data points are the average of three measurements ± standard deviation; error bars not visible are obstructed by the point. Abbreviations: SDK, mithramycin SDK; SK, mithramycin SK.

Figure 5

The cytotoxicity of free drug and nanoparticulate formulations for SK and SDK against A549 cell lines. Notes: All points are the average of four measurements ± one standard deviation; error bars not visible are obstructed by the point. Abbreviations: SDK, mithramycin SDK; SK, mithramycin SK.