Novel photo-active labeled diagnostic and therapeutic peptides which are conformationally constrained backbone cyclized somatostatin analogs, having improved somatostatin 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. Furthermore, the unique patterns of receptor subtype selectivity provide compounds having improved diagnostic and therapeutic utilities. Pharmaceutical compositions comprising the photo-active backbone cyclized somatostatin analogs, reagents for synthesizing same, and methods of using such compositions for diagnostic and therapeutic purposes including optical imaging and photodynamic therapy are also disclosed.