Polymer Architecture Alters Tissue Distribution and Enhances Cytotoxicity Profiles in Orthotopic Models of Triple Negative Breast Cancers

The efficacy of nanomedicines is dependent on their access to target sites in the body, and this in turn is affected by their size, shape and transport properties in tissue. Although there have been many studies, the ability to design nanomaterials with optimal physicochemical properties for in vivo efficacy remains a significant challenge. In particular, it is difficult to quantify the detailed effects of cancer drug delivery systems in vivo as tumour volume reduction, a commonly reported marker of efficacy, does not always correlate with cytotoxicity in tumour tissue. Here, we studied the behaviour in vivo of two specific poly(2-hydroxypropyl methacrylamide) (pHPMA) pro-drugs, with the same chemical compositions of redox-responsive backbone components and pH-sensitive linkers to the anti-cancer drug doxorubicin but with varying architectures, in this case hyperbranched and star-shaped. Evaluation of the biodistribution of these polymers following systemic injection indicated differences in the circulation time and organ distribution of the two polymers, despite their very similar hydrodynamic radii (~ 10 and 15 nm) and underlying chemistry of backbone, side-chain and pro-drug linkers. In addition, both polymers showed improved tumour accumulation in orthotopic triple-negative breast cancers in mice, and decreased accumulation in healthy tissue, as compared to free doxorubicin. Importantly, there was a significant increase in tumour accumulation for the hyper-branched polymer compared to the star polymer, suggesting a possible role for solution conformations of these materials, rather than the chemistries, in mediating their performance. The results of haematoxylin and eosin assays, and TUNEL staining indicated a higher population of apoptotic cells in the tumours for both polymer pro-drug treatments, and in turn a lower population of apoptotic cells in the heart, liver and spleen, as compared to free doxorubicin. In particular, the hyperbranched polymer demonstrated significantly higher tumour deposition and apoptosis levels than its star shaped counterpart. Taken together, these data suggest that the penetration of both of these polymer pro-drugs was enhanced in tumour tissue relative to free doxorubicin, and that the combination of size, architecture, bioresponsive backbone and drug linker degradation yielded greater efficacy for the polymers as measured by biomarkers other than that of tumour volume.