You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

A novel series of histamine H4 receptor antagonists based on the pyrido[3,2-d]pyrimidine scaffold: Comparison of hERG binding and target residence time with PF-3893787. In this work we describe the optimization of a lead compound based on the quinazoline template to give a new series of potent pyrido[3,2-d]pyrimidines as histamine H4 receptor antagonists. The pyrido[3,2-d]pyrimidine ligands have significantly reduced hERG binding compared to clinical stage compound PF-3893787 while showing good affinities at the human and rodent histamine receptors. The receptor residence time of several of these new compounds was determined for the human H4R and compared with JNJ7777120 and PF-3893787. The pyrido[3,2-d]pyrimidines showed residence times lower than JNJ7777120 but comparable to the residence time of PF-3893787. Overall, the pyrido[3,2-d]pyrimidines show an excellent in vitro profile that warrants their further investigation in relevant models of human disease.
{"Chemical": ["quinazoline", "pyrido[3,2-d]pyrimidines", "pyrido[3,2-d]pyrimidine", "JNJ7777120", "PF-3893787", "histamine"]}