Luna Liu

Conjugation site modulates the in vivo stability and therapeutic activity of antibody-drug conjugates

The reactive thiol in cysteine is used for coupling maleimide linkers in the generation of antibody conjugates. To assess the impact of the conjugation site, we engineered cysteines into a therapeutic HER2/neu antibody at three sites differing in solvent accessibility and local charge. The highly solvent-accessible site rapidly lost conjugated thiol-reactive linkers in plasma owing to maleimide exchange with reactive thiols in albumin, free cysteine or glutathione. In contrast, a partially accessible site with a positively charged environment promoted hydrolysis of the succinimide ring in the linker, thereby preventing this exchange reaction. The site with partial solvent-accessibility and neutral charge displayed both properties. In a mouse mammary tumor model, the stability and therapeutic activity of the antibody conjugate were affected positively by succinimide ring hydrolysis and negatively by maleimide exchange with thiol-reactive constituents in plasma. Thus, the chemical and structural dynamics of the conjugation site can influence antibody conjugate performance by modulating the stability of the antibody-linker interface.

Characterization of intact antibody–drug conjugates from plasma/serum in vivo by affinity capture capillary liquid chromatography–mass spectrometry

Antibody-drug conjugates (ADCs) are designed to facilitate the targeted delivery of cytotoxic drugs to improve their tumor fighting effects and minimize systemic toxicity. However, efficacy and safety can potentially be compromised due to the release of conjugated drugs from the ADC with time while in circulation, resulting in changes in the drug-to-antibody ratio (DAR). Current understanding of this process is limited because existing methods such as immunoassays fail to distinguish ADCs with different DARs. Here we demonstrate a novel method with bead-based affinity capture and capillary liquid chromatography-mass spectrometry to allow direct measurement of drug release by quantifying DAR distributions of the ADC in plasma/serum. This method successfully identified individual intact conjugated antibody species produced due to drug loss from ADCs (e.g., an engineered site-specific anti-MUC16 THIOMAB-drug conjugate) and measured the corresponding DAR distributions in vitro and in vivo. Information obtained can provide insights into the mechanisms involved in drug loss and help to optimize ADC therapeutics. Other potential applications of the method may include characterization of posttranslational modifications, protein adducts, and immunogenicity.

Characterization of the drug-to-antibody ratio distribution for antibody-drug conjugates in plasma/serum.

BACKGROUND Antibody-drug conjugates (ADCs) are a new class of cancer therapeutics that deliver potent cytotoxins specifically to tumors to minimize systemic toxicity. However, undesirable release of covalently linked drugs in circulation can affect safety and efficacy. The objective of this manuscript was to propose and assess the assays that allow for the characterization of the drug deconjugation in plasma/serum. RESULTS ADCs of three main drug conjugation platforms, linked via lysine, site-specific engineered cysteine or reduced interchain disulfide cysteine residues, were analyzed using affinity capture for sample enrichment coupled with LC-MS or hydrophobic interaction chromatography-UV for detection. These novel approaches enabled measurement of the relative abundance of individual ADC species with different drug-to-antibody ratios, while maintaining their structural integrity. CONCLUSION The characterization data generated by affinity capture LC-MS or hydrophobic interaction chromatography-UV provided critical mechanistic insights into understanding the stability and bioactivity of ADCs in vivo, and also helped the development of appropriate quantitative ELISAs.

Significant Species Difference in Amide Hydrolysis of GDC-0834, a Novel Potent and Selective Bruton's Tyrosine Kinase Inhibitor

(R)-N-(3-(6-(4-(1,4-Dimethyl-3-oxopiperazin-2-yl)phenylamino)-4-methyl-5-oxo-4,5-dihydropyrazin-2-yl)-2-methylphenyl)-4,5,6,7-tetrahydrobenzo[b]thiophene-2-carboxamide (GDC-0834) is a potent and selective inhibitor of Brutons tyrosine kinase (BTK), investigated as a potential treatment for rheumatoid arthritis. In vitro metabolite identification studies in hepatocytes revealed predominant formation of an inactive metabolite (M1) via amide hydrolysis in human. The formation of M1 appeared to be NADPH-independent in human liver microsomes. M1 was found in only minor to moderate quantities in plasma from preclinical species dosed with GDC-0834. Human clearance predictions using various methodologies resulted in estimates ranging from low to high. In addition, GDC-0834 exhibited low clearance in PXB chimeric mice with humanized liver. Uncertainty in human pharmacokinetic prediction and high interest in a BTK inhibitor for clinical evaluation prompted an investigational new drug strategy, in which GDC-0834 was rapidly advanced to a single-dose human clinical trial. GDC-0834 plasma concentrations in humans were below the limit of quantitation (<1 ng/ml) in most samples from the cohorts dosed orally at 35 and 105 mg. In contrast, substantial plasma concentrations of M1 were observed. In human plasma and urine, only M1 and its sequential metabolites were identified. The formation kinetics of M1 was evaluated in rat, dog, monkey, and human liver microsomes in the absence of NADPH. The maximum rate of M1 formation (Vmax) was substantially higher in human compared with that in other species. In contrast, the Michaelis-Menten constant (Km) was comparable among species. Intrinsic clearance (Vmax/Km) of GDC-0834 from M1 formation in human was 23- to 169-fold higher than observed in rat, dog, and monkey.

Site-Specific Trastuzumab Maytansinoid Antibody–Drug Conjugates with Improved Therapeutic Activity through Linker and Antibody Engineering

Antibody-drug conjugates (ADCs) have a significant impact toward the treatment of cancer, as evidenced by the clinical activity of the recently approved ADCs, brentuximab vedotin for Hodgkin lymphoma and ado-trastuzumab emtansine (trastuzumab-MCC-DM1) for metastatic HER2+ breast cancer. DM1 is an analog of the natural product maytansine, a microtubule inhibitor that by itself has limited clinical activity and high systemic toxicity. However, by conjugation of DM1 to trastuzumab, the safety was improved and clinical activity was demonstrated. Here, we report that through chemical modification of the linker-drug and antibody engineering, the therapeutic activity of trastuzumab maytansinoid ADCs can be further improved. These improvements include eliminating DM1 release in the plasma and increasing the drug load by engineering four cysteine residues into the antibody. The chemical synthesis of highly stable linker-drugs and the modification of cysteine residues of engineered site-specific antibodies resulted in a homogeneous ADC with increased therapeutic activity compared to the clinically approved ADC, trastuzumab-MCC-DM1.

A multiplexed hybrid LC–MS/MS pharmacokinetic assay to measure two co-administered monoclonal antibodies in a clinical study

BACKGROUND Combination therapies with monoclonal antibodies (mAbs) enhance therapeutic activity and may circumvent drug resistance. However, these studies present bioanalytical challenges for ligand-binding assays (LBAs). Recent MS-based protein quantification offers an alternative for bioanalysis. RESULTS A hybrid LC-MS/MS assay was developed to simultaneously measure human serum concentrations of two mAbs. Anti-idiotypic reagents that did not work in LBAs were successfully used for mAb affinity capture enrichment. Stable isotope-labeled peptide internal standards were employed. The mAb quantification involved measuring a signature CDR peptide derived from each mAb as a surrogate. Selected clinical samples were successfully analyzed. CONCLUSION The multiplexed LC-MS/MS method provided a powerful quantitative tool for clinical PK assessment of co-administered mAbs without the requirement for stringent affinity capture reagents.

Mechanism-Based Pharmacokinetic/Pharmacodynamic Model for THIOMAB™ Drug Conjugates

ABSTRACTPurposeTHIOMAB™ drug conjugates (TDCs) with engineered cysteine residues allow site-specific drug conjugation and defined Drug-to-Antibody Ratios (DAR). In order to help elucidate the impact of drug-loading, conjugation site, and subsequent deconjugation on pharmacokinetics and efficacy, we have developed an integrated mathematical model to mechanistically characterize pharmacokinetic behavior and preclinical efficacy of MMAE conjugated TDCs with different DARs. General applicability of the model structure was evaluated with two different TDCs.MethodPharmacokinetics studies were conducted for unconjugated antibody and purified TDCs with DAR-1, 2 and 4 for trastuzumab TDC and Anti-STEAP1 TDC in mice. Total antibody concentrations and individual DAR fractions were measured. Efficacy studies were performed in tumor-bearing mice.ResultsAn integrated model consisting of distinct DAR species (DAR0-4), each described by a two-compartment model was able to capture the experimental data well. Time series measurements of each Individual DAR species allowed for the incorporation of site-specific drug loss through deconjugation and the results suggest a higher deconjugation rate from heavy chain site HC-A114C than the light chain site LC-V205C. Total antibody concentrations showed multi-exponential decline, with a higher clearance associated with higher DAR species. The experimentally observed effects of TDC on tumor growth kinetics were successfully described by linking pharmacokinetic profiles to DAR-dependent killing of tumor cells.ConclusionResults from the integrated model evaluated with two different TDCs highlight the impact of DAR and site of conjugation on pharmacokinetics and efficacy. The model can be used to guide future drug optimization and in-vivo studies.

Chemical Structure and Concentration of Intratumor Catabolites Determine Efficacy of Antibody Drug Conjugates

Despite recent technological advances in quantifying antibody drug conjugate (ADC) species, such as total antibody, conjugated antibody, conjugated drug, and payload drug in circulation, the correlation of their exposures with the efficacy of ADC outcomes in vivo remains challenging. Here, the chemical structures and concentrations of intratumor catabolites were investigated to better understand the drivers of ADC in vivo efficacy. Anti-CD22 disulfide-linked pyrrolobenzodiazepine (PBD-dimer) conjugates containing methyl- and cyclobutyl-substituted disulfide linkers exhibited strong efficacy in a WSU-DLCL2 xenograft mouse model, whereas an ADC derived from a cyclopropyl linker was inactive. Total ADC antibody concentrations and drug-to-antibody ratios (DAR) in circulation were similar between the cyclobutyl-containing ADC and the cyclopropyl-containing ADC; however, the former afforded the release of the PBD-dimer payload in the tumor, but the latter only generated a nonimmolating thiol-containing catabolite that did not bind to DNA. These results suggest that intratumor catabolite analysis rather than systemic pharmacokinetic analysis may be used to better explain and predict ADC in vivo efficacy. These are good examples to demonstrate that the chemical nature and concentration of intratumor catabolites depend on the linker type used for drug conjugation, and the potency of the released drug moiety ultimately determines the ADC in vivo efficacy.

Modulating Therapeutic Activity and Toxicity of Pyrrolobenzodiazepine Antibody-Drug Conjugates with Self-Immolative Disulfide Linkers

A novel disulfide linker was designed to enable a direct connection between cytotoxic pyrrolobenzodiazepine (PBD) drugs and the cysteine on a targeting antibody for use in antibody–drug conjugates (ADCs). ADCs composed of a cysteine-engineered antibody were armed with a PBD using a self-immolative disulfide linker. Both the chemical linker and the antibody site were optimized for this new bioconjugation strategy to provide a highly stable and efficacious ADC. This novel disulfide ADC was compared with a conjugate containing the same PBD drug, but attached to the antibody via a peptide linker. Both ADCs had similar efficacy in mice bearing human tumor xenografts. Safety studies in rats revealed that the disulfide-linked ADC had a higher MTD than the peptide-linked ADC. Overall, these data suggest that the novel self-immolative disulfide linker represents a valuable way to construct ADCs with equivalent efficacy and improved safety. Mol Cancer Ther; 16(5); 871–8. ©2017 AACR.

Critical role of bioanalytical strategies in investigation of clinical PK observations, a Phase I case study

RG7652 is a human immunoglobulin 1 (IgG1) monoclonal antibody (mAb) targeting proprotein convertase subtilisin/kexin type 9 (PCSK9) and is designed for the treatment of hypercholesterolemia. A target-binding enzyme-linked immunosorbent assay (ELISA) was developed to measure RG7652 levels in human serum in a Phase I study. Although target-binding assay formats are generally used to quantify free therapeutic, the actual therapeutic species being measured are affected by assay conditions, such as sample dilution and incubation time, and levels of soluble target in the samples. Therefore, in the presence of high concentrations of circulating target, the choice of reagents and assay conditions can have a significant effect on the observed pharmacokinetic (PK) profiles. Phase I RG7652 PK analysis using the ELISA data resulted in a nonlinear dose normalized exposure. An investigation was conducted to characterize the ELISA to determine whether the assay format and reagents may have contributed to the PK observation. In addition, to confirm the ELISA results, a second orthogonal method, liquid chromatography tandem mass spectrometry (LC-MS/MS) using a signature peptide as surrogate, was developed and implemented. A subset of PK samples, randomly selected from half of the subjects in the 6 single ascending dose (SAD) cohorts in the Phase I clinical study, was analyzed with the LC-MS/MS assay, and the data were found to be comparable to the ELISA data. This paper illustrates the importance of reagent characterization, as well as the benefits of using an orthogonal approach to eliminate bioanalytical contributions when encountering unexpected observations.