The Pharmacologic Basis for Antibody-Auristatin Conjugate Activity

Abstract

Antibody-drug conjugates (ADCs) made with auristatin antimitotic agents have shown significant preclinical and clinical oncology activity. SGN-75 is composed of the anti-CD70 antibody h1F6 conjugated to monomethylauristatin F through a noncleavable maleimidocaproyl linkage. To understand the pharmacologic basis of the activity of this ADC, its pharmacokinetics and biodistribution were evaluated in a mouse xenograft model with use of a dual-radiolabeled ADC. The concentrations of antibody, total auristatin (conjugated plus unconjugated), and unconjugated auristatin were measured simultaneously in serum, tumor, and 16 normal tissues. Serum pharmacokinetic parameters for antibody and total auristatin were similar with very little unconjugated auristatin observed, demonstrating a high degree of stability. The kinetic values in normal tissues generally tracked with serum: the first time point (1 h) had the highest antibody and total auristatin concentrations with low unconjugated auristatin concentrations, with the exception of organs expected to be involved in hepatobiliary clearance of the ADC, where total and unconjugated auristatin concentrations peaked at 4 h and then rapidly decreased. In tumors, antibody concentrations were maximal at 1 day, with total auristatin increasing until 2 days. Intratumoral unconjugated auristatin was a substantial fraction of the total auristatin and reached concentrations much higher than in normal tissues. The exposure of the tumor to total and unconjugated auristatin was tens to hundreds times higher than normal tissue exposure. The data establish the pharmacologic basis of activity of the ADC through specific tumor targeting, intratumoral auristatin retention, and ADC stability in the systemic circulation.

Synthesis of site-specific antibody-drug conjugates using unnatural amino acids

“…Pharmacokinetic parameters were calculated by noncompartment modeling, and also did not show significant differences in clearance rates for IgG-nAF and IgG at 0.31 ± 0.03 mL/h/kg and 0.3 ± 0.11 mL/h/kg, respectively (Table S2). Thus, our site-specific ADCs have pharmacokinetics similar to the unconjugated antibody, and improved relative to many nonspecifically conjugated ADCs (38,39). It is likely that nonspecific conjugates of hydrophobic drugs lead to a population of antibodies with reduced solubility.…”

“…Next, we examined the pharmacokinetics of anti-Her2-IgG(HC-A121X)-nAF in rodents because most ADCs have reduced half-lives compared with the native antibody (20,38,39). A single dose of 1 mg/kg anti-Her2-IgG-nAF in PBS was injected intravenously in five rats, and serum was collected at regular intervals for 14 d and analyzed by ELISA using antihuman κ-antibody.…”

Antibody Drug Conjugates: Nonclinical Safety Considerations

“…Biodistribution of the unconjugated components is also significantly impacted by conjugation (5). Antibodies are largely restricted to plasma space with slow diffusion into tissues (12,13) while small molecule warheads generally have a much higher volume of distribution depending on chemical properties of the molecule. The conjugation process limits warhead to antibody-restricted plasma space and target-expressing cells (14,15).…”

Immunogenicity of Antibody Drug Conjugates: Bioanalytical Methods and Monitoring Strategy for a Novel Therapeutic Modality

“…ADCs were developed as oncology therapeutics coupling the specificity of mAbs with the cytotoxicity of a small-molecule drug (e.g., maytansinoid, calicheamicin, auristatin) (6)(7)(8)(9)(10). The ADC's mechanism of action hinges on the release of a cytotoxic payload after internalization by one of several processes into the target cell (11)(12)(13).…”