Robert R. Milburn

Monitoring chemical reactions by low-field benchtop NMR at 45 MHz: pros and cons.

Monitoring chemical reactions is the key to controlling chemical processes where NMR can provide support. High‐field NMR gives detailed structural information on chemical compounds and reactions; however, it is expensive and complex to operate. Conversely, low‐field NMR instruments are simple and relatively inexpensive alternatives. While low‐field NMR does not provide the detailed information as the high‐field instruments as a result of their smaller chemical shift dispersion and the complex secondary coupling, it remains of practical value as a process analytical technology (PAT) tool and is complimentary to other established methods, such as ReactIR and Raman spectroscopy. We have tested a picoSpin‐45 (currently under ThermoFisher Scientific) benchtop NMR instrument to monitor three types of reactions by 1D 1H NMR: a Fischer esterification, a Suzuki cross‐coupling, and the formation of an oxime. The Fischer esterification is a relatively simple reaction run at high concentration and served as proof of concept. The Suzuki coupling is an example of a more complex, commonly used reaction involving overlapping signals. Finally, the oxime formation involved a reaction in two phases that cannot be monitored by other PAT tools. Here, we discuss the pros and cons of monitoring these reactions at a low‐field of 45 MHz by 1D 1H NMR. Copyright

SLC46A3 Is Required to Transport Catabolites of Noncleavable Antibody Maytansine Conjugates from the Lysosome to the Cytoplasm

Antibody-drug conjugates (ADC) target cytotoxic drugs to antigen-positive cells for treating cancer. After internalization, ADCs with noncleavable linkers are catabolized to amino acid-linker-warheads within the lysosome, which then enter the cytoplasm by an unknown mechanism. We hypothesized that a lysosomal transporter was responsible for delivering noncleavable ADC catabolites into the cytoplasm. To identify candidate transporters, we performed a phenotypic shRNA screen with an anti-CD70 maytansine-based ADC. This screen revealed the lysosomal membrane protein SLC46A3, the genetic attenuation of which inhibited the potency of multiple noncleavable antibody-maytansine ADCs, including ado-trastuzumab emtansine. In contrast, the potencies of noncleavable ADCs carrying the structurally distinct monomethyl auristatin F were unaffected by SLC46A3 attenuation. Structure-activity experiments suggested that maytansine is a substrate for SLC46A3. Notably, SLC46A3 silencing led to relative increases in catabolite concentrations in the lysosome. Taken together, our results establish SLC46A3 as a direct transporter of maytansine-based catabolites from the lysosome to the cytoplasm, prompting further investigation of SLC46A3 as a predictive response marker in breast cancer specimens.

Altering Antibody–Drug Conjugate Binding to the Neonatal Fc Receptor Impacts Efficacy and Tolerability

Antibody-drug conjugates (ADC) rely on the target-binding specificity of an antibody to selectively deliver potent drugs to cancer cells. IgG antibody half-life is regulated by neonatal Fc receptor (FcRn) binding. Histidine 435 of human IgG was mutated to alanine (H435A) to explore the effect of FcRn binding on the pharmacokinetics, efficacy, and tolerability of two separate maytansine-based ADC pairs with noncleavable linkers, (c-DM1 and c-H435A-DM1) and (7v-Cys-may and 7v-H435A-Cys-may). The in vitro cell-killing potency of each pair of ADCs was similar, demonstrating that H435A showed no measurable impact on ADC bioactivity. The H435A mutant antibodies showed no detectable binding to human or mouse FcRn in vitro, whereas their counterpart wild-type IgG ADCs were found to bind to FcRn at pH = 6.0. In xenograft bearing SCID mice expressing mouse FcRn, the AUC of 7v-Cys-may was 1.6-fold higher than that of 7v-H435A-may, yet the observed efficacy was similar. More severe thrombocytopenia was observed with 7v-H435A-Cys-may as compared to 7v-Cys-may at multiple dose levels. The AUC of c-DM1 was approximately 3-fold higher than that of c-H435A-DM1 in 786-0 xenograft bearing SCID mice, which led to a 3-fold difference in efficacy by dose. Murine FcRn knockout, human FcRn transgenic line 32 SCID animals bearing 786-0 xenografts showed an amplified exposure difference between c-DM1 and c-H435A-DM1 as compared to murine FcRn expressing SCID mice, leading to a 10-fold higher dose required for efficacy despite a 6-fold higher AUC of the c-H435A-DM1. The accelerated clearance observed for the noncleavable maytansine ADCs with the H435A FcRn mutation led to reduced efficacy at equivalent doses and exacerbation of clinical pathology parameters (decreased tolerability) at equivalent doses. The results show that reduced ADC clearance mediated by FcRn modulation can improve therapeutic index.