Finding the optimal balance: challenges of improving conventional cancer chemotherapy using suitable combinations with nano-sized drug delivery systems.

Anticancer drugs as well as nano-sized drug delivery systems face many barriers that hinder penetration deeply and evenly into solid tumors: a chaotic, tortuous vascular compartment resulting in tumor tissue distant from microvessels, a heterogeneous blood flow distribution with a concomitant defective microcirculatory exchange process, and a high interstitial fluid pressure. Furthermore, a resulting hostile tumor microenvironment characterized by hypoxia and/or extracellular acidosis can reduce the efficacy of anticancer drugs and confer drug resistance. Conversely, the enhanced permeation and retention effect has become the gold standard for developing macromolecular prodrugs and nano-sized drug delivery systems. Preclinically, there are meanwhile numerous in vivo proof-of-concepts that demonstrate not only a better tolerability of nano-sized drug delivery systems but also of enhanced antitumor efficacy compared to the conventional clinical standard. When faced with such a complex and heterogeneous disease as cancer in humans, it is more likely that a tailor-made combination of different therapeutic strategies will achieve the best results. In this respect, combining low-molecular weight cytostatic drugs with nano-sized drug delivery systems appears to be a natural choice for combination therapy that aims at distributing anticancer drugs at higher concentrations in the tumor in a more even manner. To date, such drug delivery approaches have been inadequately explored. In this review, we summarize the state-of-the-art of combination approaches with liposomal doxorubicin (Doxil™), the paclitaxel-albumin nanoparticle (Abraxane™) and the albumin-binding doxorubicin prodrug DOXO-EMCH (INNO-206), and discuss the insights obtained and perspectives for further research in this intriguing and promising field of drug delivery research.