Analogs of SRIF which are selective for SSTR4 in contrast to the other cloned SRIF receptors are useful in determining tissue and cellular expression of the
receptor SSTR4 and its biological role in regulating
tumor growth. SRIF analog peptides, such as des-AA1,2,4,5,12,13[Ala7]-SRIF; des-AA1,2,4,5,12,13[Aph7]-SRIF, des-AA1,2,4,5,12,13[Aph7]Cbm-SRIF; des-AA1,2,4,5,12,13[Tyr2,Ala7]-Cbm-SRIF, and des-AA1,2,4,5,12,13[Tyr7,CβMe-L-2Nal8]-SRIF, and counterparts incorporating D-Cys3 and / or D-Trp8 and / or Ala11, bind with high affinity to the cloned human
receptor SSTR4 and activate the
receptor, but they do not bind with significant affinity to human SSTR1, SSTR2, SSTR3 or SSTR5. By incorporating an iodinated
tyrosine in position-2 in these SSTR4-selective SRIF analogs, a labeled compound useful in
drug-screening methods is provided. Alternatively, for use in therapy, cytotoxins or highly radioactive elements can be N-terminally coupled or complexed thereto.