A compound of the invention is represented by the formula: ##STR1## wherein R.sup.1 and R.sup.2 are the same or different, an acyl group; R.sup.3, R.sup.4, R.sup.6 and R.sup.7 are the same or different, hydrogen or an alkyl group; R.sup.5 is hydrogen, an alkyl group or a hydroxyl group which may optionally be protected, or R.sup.4 and R.sup.5 are combined to form a chemical bond; X is a carbonyl group or a sulfonyl group; Y is an amino acid sequence consisting of 1 to 7 amino acid residues which may optionally be protected and having optionally an intervening --SO.sub.2 NH--; n is an integer of 0 to 2, or a salt thereof. The compound or salt thereof of the present invention possesses immunoenhancing activity and platelet reduction recovery activity, and can be used as a prophylactic/therapeutic agent for leukocytopenia in cancer chemotherapy or radiotherapy, as an immunoenhancing agent in bone marrow transplantation therapy and the treatment of osteomyelodysplasia and aplastic anemia, and as a prophylactic/therapeutic agent for thrombocytopenia.
A series of TAN-1511 analogues bearing a non-peptide spacer in place of the Gly-Gly-Gly sequence in the peptide moiety was synthesized, and the effects of these compounds on the proliferation of bone marrow cells in culture and experimental leukocytopenia in mice were examined. The structure-activity relationships obtained were as follows. As the substituent at the 2-position of the 4-thiaheptanoic acid framework, an amino group, methyl group or hydrogen was preferable; as a spacer in place of the Gly-Gly-Gly sequence, a 4-aminobenzoyl or 4-aminomethylbenzoyl group was suitable; and as the fatty acids bonded to the 6, 7-dihydroxy groups, C16 fatty acid was best. Compounds 12f, 30d and 30i potently promoted the proliferation of bone marrow cells in culture and the restoration of leukocyte counts in a murine leukocytopenia model.