Megapinosome: Morphological description of a novel organelle

Megapinosomes and homologous structures in hematopoietic cells

Megapinosomes are endocytic organelles found in human macrophage colony-stimulating factor (M-CSF) monocyte-derived M macrophages. They are large (several microns) and have a complex internal structure that is connected with the cytosol and consists of interconnected knots and concave bridges with sizes in the range of 100 nm. We called this structure trabecular meshwork. The luminal part of the megapinosome can be connected with luminal tubules and cisterns that form the megapinosome complex. The structures are especially well visible in scanning electron tomography when macrophages are prepared by high-pressure freezing and freeze substitution. Our research received a new impulse after studying the literature on hematopoietic cells, where very similar, most likely homologous, structures have been published in peritoneal macrophages as well as in megakaryocytes and blood platelets. In platelets, they serve as membrane storage that is used for structural changes of platelets during activation.

Stepping forward in antibody-drug conjugate development

Antibody-drug conjugates (ADCs) are cancer therapeutic agents comprised of an antibody, a linker and a small-molecule payload. ADCs use the specificity of the antibody to target the toxic payload to tumor cells. After intravenous administration, ADCs enter circulation, distribute to tumor tissues and bind to the tumor surface antigen. The antigen then undergoes endocytosis to internalize the ADC into tumor cells, where it is transported to lysosomes to release the payload. The released toxic payloads can induce apoptosis through DNA damage or microtubule inhibition and can kill surrounding cancer cells through the bystander effect. The first ADC drug was approved by the United States Food and Drug Administration (FDA) in 2000, but the following decade saw no new approved ADC drugs. From 2011 to 2018, four ADC drugs were approved, while in 2019 and 2020 five more ADCs entered the market. This demonstrates an increasing trend for the clinical development of ADCs. This review summarizes the recent clinical research, with a specific focus on how the in vivo processing of ADCs influences their design. We aim to provide comprehensive information about current ADCs to facilitate future development.