the subject matter of the present invention is a cyclic or linear tetra - or pentapeptide binding to somatostatin receptors . the compounds of the invention are characterised in that they contain the radical of an amino carboxylic acid bearing a five - membered ring in the peptide backbone which may optionally contain o , s , se , n , or p . these compounds are easy to prepare and display increased stability against peptidases . the compounds of the present invention induce apoptosis of tumour cells and the use of said compounds for cancer therapy is described . in particular , the compounds are characterised in that they are active even against tumour cells displaying resistance against other somatostatin derivatives such as octreotide . in addition , the use of the compounds of the invention for tumour diagnosis by means of positron - emission tomography is described , as well as their use as agents against neurogenic inflammation .