Novel diagnostic and therapeutic peptides which are somatostatin analogs backbone cyclized through metal complexation, having improved somatostin receptor subtype affinity and selectivity are disclosed. The backbone cyclized peptide analogs disclosed possess unique and superior properties over other analogs, such as chemical and metabolic stability, selectivity, increased bioavailability and improved pharmacokinetics. Pharmaceutical compositions comprising the backbone cyclized somatostin analogs and radiolabelled analogs, reagents for synthesizing same, and methods of using such compositions for diagnostic and therapeutic purposes are also disclosed.