The invention provides novel prostaglandin analogs. In particular, the present invention relates to compounds having a structure according to formula (I) wherein R1, R2, R3, R4, R5, R6, W, X, Z, a, b, p and q are defined below. This invention also includes optical isomers, diastereomers and enantiomers of formula (I), and pharmaceutically-acceptable salts, biohydrolyzable amides, esters, and imides thereof. The compounds of the present invention are useful for the treatment of a variety of diseases and conditions, such as bone disorders and glaucoma. Accordingly, the invention further provides pharmaceutical compositions comprising these compounds. The invention still further provides methods of treatment for bone disorders and glaucoma using these compounds or the compositions containing them.