Cancer remains a leading cause of death in companion animals. In human medicine, liposomes and nanoparticles have been extensively investigated as drug delivery systems (DDS) for anticancer agents due to their ability to target cancerous cells and reduce the negative side effects of free cytostatic drugs. In this review, the authors discuss the results of clinical trials using liposomes and polymer-based nanoparticles as DDS to improve cancer treatment in dogs and cats, indicating which ones

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... m worth further evaluation. The authors then overview ongoing animal cancer clinical trials, evaluating nano-DDS registered on the American Veterinary Medical Association Animal Health Studies Database. Finally, the authors indicate the nano-drugs that require further in vivo evaluation based on the encouraging results obtained from in vitro studies. Conclusions: Liposomes have been the most investigated nano-DDS in veterinary medicine. The lack of cardiotoxicity of the commercially available liposomal doxorubicin (Doxil/Caelyx) suggests it should be used in dogs with cardiac disorders, rather than using free doxorubicin. Cisplatin-incorporated hyaluronic acid nanoparticles, nanocrystals of cisplatin, and paclitaxel are the most promising nano-drugs for potent applications in treating various canine cancers (e.g., oral melanoma, oral sarcoma, and anal gland adenocarcinoma) and their translation into the treatment of human diseases. (P-gp), one of the main efflux pumps responsible for MDR, which can cause chemotherapy failure [8] . Various drug delivery systems (DDS) for anticancer agents such as liposomes, micelles, and albumin-, metal-, and polymer-based nanoparticles were reported to improve the stability of hydrophobic drugs, increase accumulation in cancer tissue, and therefore reduce negative side effects [5] [6] [7] [9] [10] [11] [12] [13] . In human medicine, there are few nano-drugs registered in the USA and Europe for cancer treatment, and many others are currently being tested in clinical trials [14] . Nanooncology is a new field in veterinary medicine, with only a few clinical and preclinical studies investigating nano-drugs (Table 1) and their potential application in pet animal cancer treatments, some of which were first performed as translational research for application in human disease. Dogs with spontaneous tumours are good models for human cancer studies, resulting in their use in biodistribution, pharmacokinetic, and efficacy studies on novel anticancer drugs. According to the American Veterinary Medical Association (AVMA) website of animal cancer clinical trials (https://ebusiness.avma.org/aahsd/study_search.aspx), there are currently six clinical trials registered with the use of nanoparticles as DDS to improve anticancer therapy in dogs: nanocrystals of cisplatin (studies No. AAHSD000024, AAHSD000176, AAHSD000370, AAHSD004339), paclitaxel (study No. AAHSD000021), and IL-12 (study No. AAHSD000445) (data collected in August 2017). Each of these studies also has a translational value, as solid tumours arising spontaneously in dogs share many similarities with the same types of neoplasms in humans, for example, canine osteosarcoma (OSA) with paediatric osteosarcomas [15] [16] [17] . As a result, human cancer patients indirectly benefit from advancing our knowledge of cancer therapy in dogs. Therefore, these studies may be used as proof of concepts and the basis for developing novel anticancer drugs for people. The main purpose of this review is to summarize the results of clinical trials on the use of nano-carriers to improve anticancer therapy in dogs and cats, indicating the most promising ones. Moreover, the authors present ongoing animal cancer clinical trials on nano-DDS registered in AVMA Animal Health Studies Database and indicate the nano-drugs that need to be further evaluated in vivo based on the encouraging results of in vitro studies using canine and feline cancer cell lines. Search Methodology This review is based on a search of the PubMed database (http://www.ncbi.nlm.nih.gov/pubmed) using the terms "canine" or "feline" and "nanoparticles" or "liposomes" and "cancer". This review is a synthesis of the current use of liposomes and nanoparticles as DDS in cancer treatments for companion animals and highlights those that warrant further investigation.