The hydrazone linker's instability in drug conjugates here as elsewhere, compelled resorting to a more stable cleavable linker than that used in the initial hydrazone linkers of the above NInC hybrids. Inasmuch as Besponsa ® i.e. inotuzumab ozogamicin conjugate owes its clinical success to its somewhat stable carbohydrazone linker in the systemic circulation, this served as an inspiration for designing new NInC hybrids, featuring slightly more stable carbohydrazide linkers than the previously designed acid/base labile hydrazone linkers. Knowing that the oxidation level of carbon in a carbohydrazide is three, and that of 1 is two, synthesis of 28 from 5-nitro-2-furoyl chloride in a manner similar to that for 1 was attempted; however, 5-nitro-2-furoyl chloride proved too reactive such that ester 29 (ESI S35 †) had to be synthesised first, which in turn was converted to 28 in excellent yields i.e. 85-90% after flash column chromatography, and ultra-high purity i.e. >98%, as evidenced by H NMR (ESI S51 †). Unfortunately, however, despite numerous experiments carried out under a variety of conditions (Scheme 7), the Buchwald amination step, namely, the coupling of InC esters to nitrofuran 28, could not be accomplished, and hence this route to formation of stable NInC hybrids had to be abandoned.