Antiangiogenic

The tumour vasculature plays an important role in tumour growth and metastasis. Tumour angiogenesis provides more oxygen and nutrients to growing tumour cells, is not as tightly regulated as embryonic angiogenesis, and do not follow any hierarchically ordered pattern. The heterogeneity of the vasculature, high interstitial fluid pressure, poor extravasation due to sluggish blood flow, and larger distances between exchange vessels are potential barriers to the delivery of therapeutic agents to tumours. The prevention of angiogenesis, normalization of tumour vasculature, and enhancement of blood perfusion through the use of monoclonal antibodies against receptor proteins that are overexpressed on proangiogenic tumour cells, and improved, tumour-targeted delivery of therapeutic agents can all be achieved using nanocarriers of appropriate size. Nanomedicines such as polymeric nanoparticles, lipid nanoparticles, micelles, mesoporous silica particles, metal nanoparticles, noisomes, and liposomes have been developed for the delivery of anticancer drugs in combination with antiangiogenic agents. Amongst them, liposomal delivery systems are mostly approved by the FDA for clinical use. In this review, the molecular pathways of tumour angiogenesis, the physiology of tumour vasculature, barriers to tumour-targeted delivery of therapeutic agents, and the different strategies to overcome these barriers are discussed.

Tyrosine kinase inhibitors (TKIs) are molecules that compete with ATP on tyrosine kinase receptors (TKRs), blocking tyrosine kinase (TK) activation and then oncogenic pathways; they have been studied, and some of them are right approved for the treatment of many types of cancer. Among TKIs, one of the most explored chemical template is the pyrazolo[3,4-d]pyrimidine (PP) heterocyclic core, which proved to be a useful scaffold for the obtainment of effective compounds. Actually, derivatives belonging to this structural class show a large spectrum of activity, thus standing out as multi-target agents. Different PP compounds have been shown to act as: a) ABL inhibitors and antiproliferative agents against human leukemia cell lines; b) Src kinase inhibitors in neuroblastoma, medulloblastoma and osteosarcoma; c) Phospholipase D inhibitors in different neoplasias; d) Urokinase plasminogen activator inhibitors, in breast cancer. In thyroid cancer (TC), PP1 and PP2 (inhibitors of RET, Hck, lck, and fynT kinases, and a good inhibitor of c-Src and platelet-derived growth factor receptor) showed antineoplastic actvity in human papillary TC cell lines that carry spontaneous RET/PTC1 rearrangements. More recently, new derivatives, (R)-1-phenethyl-N-(1-phenylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4- amine, namely, CLM3 and CLM29, have been demonstrated to exert a multiple signal transduction inhibition (including the RET-TK, BRAF, EGFR, and with antiangiogenic activity), showing antineoplastic activity, in vitro and in vivo, in papillary dedifferentiated, medullary and anaplastic TC. These data have shown the antineoplastic activity of PP in different neoplasias, opening the way to a future clinical evaluation in human cancers.