The linker component of
antibody-drug conjugates (ADC) is a key feature in developing optimized therapeutic agents that are highly active at well tolerated doses. For maximal intratumoral
drug delivery, linkers are required that are highly stable in the systemic circulation, yet allow for efficient drug release at the target site. In this respect,
amide bond-based technologies constitute a technological advancement, since the linker half-lives in circulation ( t 1/2 approximately 7 days) are much longer than earlier generation linkers that break down within 1-2 days. The
amide linkers, some of which contain
peptides, are appended to the mAb carriers through
thioether/
maleimide adducts. Here, we describe that use of a bromoacetamidecaproyl (bac) in place of the maleimidocaproyl (mc) increases the plasma stability of resulting
thioether ADCs. One such ADC, 1F6-C4v2-bac-MMAF, which is directed against the
CD70 antigen on
lymphomas and
renal cell carcinoma, was prepared containing a bac
thioether spacer between the
drug (MMAF) and the mAb carrier (1F6-C4v2). There was no measurable systemic drug release from this ADC for 2 weeks postadministration in mice. In order to assess the impact of improving linker stability beyond mc containing ADCs, a series of mc and bac-linked 1F6-MMAF conjugates were compared for tolerability, intratumoral
drug delivery, and therapeutic efficacy in nude mice with
renal cell carcinoma xenografts. There were no statistically significant efficacy differences between sets of mc and bac containing ADCs, although the bac linker technology led to 25% higher intratumoral
drug exposure over
a 7 day period compared to the corresponding mc linker. The mechanism of drug release from
maleimide-adducts likely involves a retro-Michael reaction that takes place in plasma, based on in vitro studies demonstrating that some of the released
drug-
maleimide derivative became covalently bound to cysteine-34 of
serum albumin. In summary, the data indicate that new linkers can be obtained with improved in vivo stability by replacing the
maleimide with an
acetamide, but the resulting ADCs had similar tolerability and activity profiles.