Talisomycin B

Tallysomycins (TLMs) belong to the bleomycin family of anticancer antibiotics. TLMs differ from bleomycins primarily by the presence of a 4-amino-4,6-dideoxy-l-talose sugar attached to C-41 as part of a glycosylcarbinolamide. We previously proposed, on the basis of bioinformatics analysis of the tlm biosynthetic gene cluster from Streptoalloteichus hindustanus E465-94 ATCC 31158, that the tlmK gene is responsible for the attachment of this sugar moiety. We now report that inactivation of tlmK in S. hindustanus abolished TLM A and TLM B production, the resultant DeltatlmK mutant instead accumulated five new metabolites, and introduction of a functional copy of tlmK to the DeltatlmK mutant restored TLM A and TLM B production. Two major metabolites, TLM K-1 and TLM K-2, together with three minor metabolites, TLM K-3, TLM K-4, and TLM K-5, were isolated from the DeltatlmK mutant, and their structures were elucidated. These findings provide experimental evidence supporting the previous functional assignment of tlmK to encode a glycosyltransferase and unveil two carbinolamide pseudoaglycones as key intermediates in the TLM biosynthetic pathway. TlmK stabilizes the carbinolamide intermediates by glycosylating their hemiaminal hydroxyl groups, thereby protecting them from hydrolysis during TLM biosynthesis. In the absence of TlmK, the carbinolamide intermediates fragment to produce an amide TLM K-1 and aldehyde intermediates, which undergo further oxidative fragmentation to afford carboxylic acids TLM K-2, TLM K-3, TLM K-4, and TLM K-5.

The potency of tallysomycin S(10b) (TLM S(10b)) an analogue bleomycin was enhanced by up to 875-fold when it was conjugated to the internalizing antibody BR96. Attachment to the antibody is achieved via a Cathepsin B cleavable linker. The enhancement in potency is believed to be a result of cellular uptake of the conjugate upon antigen binding followed by rapid release of the drug inside the lysosome. This method provides a novel approach for increasing the potency and therapeutic index of nominally moderately-active cytotoxic agents.

The tumour affinity of the radioactive cobalt chelate of tallysomycin S10b (Tlm S10b), a promising structural analogue of Bleomycin, was assessed using a mouse lymphoma model. The highest concentrations (%dose/gram tissue) were observed in the kidneys, followed by the tumour (4.1%/g 1 h p.i.) at 1 h, 4 h and 48 h. The tumour had the highest concentration among all tissues at 24 h. The biodistribution profile of 60Co-Tlm S10b was distinctly different from that obtained with 60CoCl2, demonstrating the in vivo stability of the chelate. More than half of the chelate (56.8% of the injected dose) was excreted in the urine at 1 h. The highest tumour/non-tumour ratios were obtained for blood (41.3, 4 h), bone (30.5, 4 h) and muscle (29.2, 48 h). Scintigraphy at 24 h, using 57Co-Tlm S10b, showed the tumour, liver, kidney and bladder clearly. The similarities and differences exhibited by Co-Tlm S10b with reference to the literature on cobalt chelates of Bleomycin and naturally occurring tallysomycin (A + B) are discussed.