Macro-, micro- and nanoscale drug delivery systems in oncotherapy: current clinical status

In the context of an increasing cancer burden, drug delivery systems (DDSs) emerged as promising alternatives to stand-alone drugs. This review aimed to identify clinically approved oncologic DDSs and briefly discuss aspects relevant to the clinical landscape. Sixty oncologic DDSs that received regulatory approval are included, with an inclusion criterion being the existence of information on length scale. Primary data sources include journal articles and gray literature. Secondary data sources include books and review articles. Early on, ambiguities regarding key oncology and drug delivery terms are tackled. Then, the length scales (macro-, micro- and nano-) and delivery phenomena are delineated. Four targeting mechanisms are presented. No oncologic DDS capable of active targeting has been approved. The DDSs approved for oncology are evaluated, with emphasis on their chemistry and clinical performance. Using two geometrical methods, new data on the size of nineteen molecular medicines is provided. The reasons behind the five withdrawals known for oncologic DDSs are investigated. Challenges for oncologic DDSs are identified. Strategies to improve tumor targeting are systematized. Alternative applications in cancer therapy are suggested. The success of an oncologic DDS depends on whether therapeutic, manufacturing and economic requirements are met. Though the trend of oncology approvals favors the nanoscale in absolute numbers, relative numbers favor the micro- and macroscales. Proposals made to improve the rate of clinical translation are to test the clinical benefit of promising, yet insufficiently investigated, designs, and to reorient those DDSs that failed in clinical trials toward new avenues in oncotherapy.