Splenopentin (SP-5, Arg-Lys-Glu-Val-Tyr) and thymopentin (TP-5, Arg-Lys-Asp-Val-Tyr) are synthetic immunomodulating peptides corresponding to the region 32-34 of a splenic product called splenin (SP) and the thymic hormone thymopoietin (TP), respectively. TP was originally isolated as a 5-kDa (49-amino acids) protein from bovine thymus while studying effects of the thymic extracts on neuromuscular transmission and was subsequently observed to affect T cell differentiation and function. TP I and II are two closely related polypeptides isolated from bovine thymus. A radioimmunoassay for TP revealed a crossreaction with a product found in spleen and lymph node. This product, named splenin, differs from TP only in position 34, aspartic acid for bovine TP and glutamic acid for bovine splenin and it was called TP III as well. Synthetic pentapeptides (TP-5) and (SP-5), reproduce the biological activities of TP and SP, respectively. It is now evident that various forms of TPs were created by proteolytic cleavage of larger proteins during isolation, cDNA clones have been isolated for three alternatively spliced mRNAs that encodes three distinct human T cell TPs. The immunomodulatory properties of TP, SP, TP-5, SP-5 and some of their synthetic analogs reported in the literature have been briefly reviewed.

Singh, V. K., Biswas, S., Mathur, K. B., Haq, W., Garg, S. K., & Agarwal, S. S. (1998). Thymopentin and splenopentin as immunomodulators. Immunologic research, 17(3), 345-368.

Splenopentin, Arg-Lys-Glu-Val-Tyr (SP-5) and its synthetic analogs; Arg-d-Lys-Glu-Val-Tyr (pentapeptide 1), Lys-Lys-Glu-Val-Tyr (2), d-Lys-Lys-Glu-Val-Tyr (3), Arg-Lys-Gly-Val-Tyr (4), and Arg-Lys-Gln-Val-Tyr (5) have been examined for augmentation of human natural killer (NK) cell activity and human T-cell transformation response. Pentapeptides 2 and 3 were found to significantly augment the in vitro human NK cell activity. However, none of them had any effect on lymphocyte proliferative responses.

Rastogi, A., Singh, V. K., Biswas, S., Haq, W., Mathur, K. B., & Agarwal, S. S. (1993). Augmentation of human natural killer cells by splenopentin analogs. FEBS letters, 317(1-2), 93-95.