novel compounds , or pharmaceutically - acceptable salts , tautomers or prodrugs thereof , of formula i wherein a , w , x , z , r 1 - r 3 , and r 8 are as defined in the specification , are provided . also provided are methods of treating or preventing a melanin concentrating hormone - mediated disorder in a subject , comprising administering to a subject in need of such treatment or prevention a compound of formula i .