Novel 4-aminopyrimidine-5-thione derivatives are disclosed. These compounds inhibit cyclin-dependent kinases, in particular Cdk1, Cdk2 and Cdk4. These compounds and their pharmaceutically acceptable salts have antiproliferative activity and are useful in the treatment or control of cancer, in particular solid tumors. This invention is also directed to pharmaceutical compositions containing such compounds and to methods of treating or controlling cancer, most particularly the treatment or control of breast, lung, colon and prostate tumors.