Nicole M. Okeley

Antibody–drug conjugates: targeted drug delivery for cancer

The antibody-drug conjugate field has made significant progress recently owing to careful optimization of several parameters, including mAb specificity, drug potency, linker technology, and the stoichiometry and placement of conjugated drugs. The underlying reason for this has been obtained in pre-clinical biodistribution and pharmacokinetics studies showing that targeted delivery leads to high intratumoral free drug concentrations, while non-target tissues are largely spared from chemotherapeutic exposure. Recent developments in the field have led to an increase in the number of ADCs being tested clinically, with 3 in late stage clinical trials: brentuximab vedotin (also referred to as SGN-35) for Hodgkin lymphoma; Trastuzumab-DM1 for breast cancer; and Inotuzumab ozogamicin for non-Hodgkin lymphoma. This review highlights the recent pre-clinical and clinical advances that have been made.

Intracellular Activation of SGN-35, a Potent Anti-CD30 Antibody-Drug Conjugate

Purpose: SGN-35 is an antibody-drug conjugate (ADC) containing the potent antimitotic drug, monomethylauristatin E (MMAE), linked to the anti-CD30 monoclonal antibody, cAC10. As previously shown, SGN-35 treatment regresses and cures established Hodgkin lymphoma and anaplastic large cell lymphoma xenografts. Recently, the ADC has been shown to possess pronounced activity in clinical trials. Here, we investigate the molecular basis for the activities of SGN-35 by determining the extent of targeted intracellular drug release and retention, and bystander activities. Experimental Design: SGN-35 was prepared with 14C-labeled MMAE. Intracellular ADC activation on CD30+ and negative cell lines was determined using a combination of radiometric and liquid chromatograhpy/mass spectrometry-based assays. The bystander activity of SGN-35 was determined using mixed tumor cell cultures consisting of CD30+ and CD30− lines. Results: SGN-35 treatment of CD30+ cells leads to efficient intracellular release of chemically unmodified MMAE, with intracellular concentrations of MMAE in the range of 500 nmol/L. This was due to specific ADC binding, uptake, MMAE retention, and receptor recycling or resynthesis. MMAE accounts for the total detectable released drug from CD30+ cells, and has a half-life of retention of 15 to 20 h. Cytotoxicity studies with mixtures of CD30+ and CD30− cell lines indicated that diffusible released MMAE from CD30+ cells was able to kill cocultivated CD30− cells. Conclusions: MMAE is efficiently released from SGN-35 within CD30+ cancer cells and, due to its membrane permeability, is able to exert cytotoxic activity on bystander cells. This provides mechanistic insight into the pronounced preclinical and clinical antitumor activities observed with SGN-35. Clin Cancer Res; 16(3); 888–97

A Potent Anti-CD70 Antibody-Drug Conjugate Combining a Dimeric Pyrrolobenzodiazepine Drug with Site-Specific Conjugation Technology

A highly cytotoxic DNA cross-linking pyrrolobenzodiazepine (PBD) dimer with a valine-alanine dipeptide linker was conjugated to the anti-CD70 h1F6 mAb either through endogenous interchain cysteines or, site-specifically, through engineered cysteines at position 239 of the heavy chains. The h1F6239C-PBD conjugation strategy proved to be superior to interchain cysteine conjugation, affording an antibody-drug conjugate (ADC) with high uniformity in drug-loading and low levels of aggregation. In vitro cytotoxicity experiments demonstrated that the h1F6239C-PBD was potent and immunologically specific on CD70-positive renal cell carcinoma (RCC) and non-Hodgkin lymphoma (NHL) cell lines. The conjugate was resistant to drug loss in plasma and in circulation, and had a pharmacokinetic profile closely matching that of the parental h1F6239C antibody capped with N-ethylmaleimide (NEM). Evaluation in CD70-positive RCC and NHL mouse xenograft models showed pronounced antitumor activities at single or weekly doses as low as 0.1 mg/kg of ADC. The ADC was tolerated at 2.5 mg/kg. These results demonstrate that PBDs can be effectively used for antibody-targeted therapy.

The Pharmacologic Basis for Antibody-Auristatin Conjugate Activity

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.

Potent antitumor activity of the anti-CD19 auristatin antibody-drug conjugate hBU12-vcMMAE against rituximab sensitive and resistant lymphomas

Despite major advances in the treatment of non-Hodgkin lymphoma (NHL), including the use of chemotherapeutic agents and the anti-CD20 antibody rituximab, the majority of patients eventually relapse, and salvage treatments with non-cross-resistant compounds are needed to further improve patient survival. Here, we evaluated the antitumor effects of the microtubule destabilizing agent monomethyl auristatin E (MMAE) conjugated to the humanized anti-CD19 antibody hBU12 via a protease-sensitive valine-citrulline (vc) dipeptide linker. hBU12-vcMMAE induced potent tumor cell killing against rituximab-sensitive and -resistant NHL cell lines. CD19 can form heterodimers with CD21, and high levels of CD21 were reported to interfere negatively with the activity of CD19-targeted therapeutics. However, we observed comparable internalization, intracellular trafficking, and drug release in CD21(low) and CD21(high), rituximab-sensitive and -refractory lymphomas treated with hBU12-vcMMAE. Furthermore, high rates of durable regressions in mice implanted with these tumors were observed, suggesting that both rituximab resistance and CD21 expression levels do not impact on the activity of hBU12-vcMMAE. Combined, our data suggest that hBU12-vcMMAE may represent a promising addition to the treatment options for rituximab refractory NHL and other hematologic malignancies, including acute lymphoblastic leukemia.

Anti-CD30 diabody-drug conjugates with potent antitumor activity

Anti-CD30 diabodies were engineered with two cysteine mutations for site-specific drug conjugation in each chain of these homodimeric antibody fragments. Diabodies were conjugated with ∼4 equivalents of the anti-tubulin drugs, monomethyl auristatin E or F, via a protease-cleavable dipeptide linker, to create the conjugates, diabody-vcE4 and diabody-vcF4, respectively. Diabody conjugation had only minor (<3-fold) effects on antigen binding. Diabody-vcF4 was potently cytotoxic against the antigen-positive cell lines, Karpas-299 (34 pmol/L IC50) and L540cy (22 pmol/L IC50), and was 8- and 21-fold more active than diabody-vcE4 against these cell lines, respectively. Clearance of diabody-vcF4 (99-134 mL/d/kg) was 5-fold slower than for the nonconjugated diabody in naive severe combined immunodeficient mice. Diabody-vcF4 had potent and dose-dependent antitumor activity against established Karpas-299 xenografts and gave durable complete responses at well-tolerated doses. Biodistribution experiments with diabody-[3H]-vcF4 (0.72-7.2 mg/kg) in tumor-bearing mice showed a dose-dependent increase in total auristatin accumulation in tumors (≤520 nmol/L) and decrease in relative auristatin accumulation (≤8.1 %ID/g), with peak localization at 4 to 24 h after dosing. Diabody-vcF4 had ∼4-fold lower cytotoxic activity than the corresponding IgG1-vcF4 conjugate in vitro. A similar potency difference was observed in vivo despite 25- to 34-fold faster clearance of diabody-vcF4 than IgG1-vcF4. This may reflect that dose-escalated diabody-vcF4 can surpass IgG1-vcF4 in auristatin delivery to tumors, albeit with higher auristatin exposure to some organs including kidney and liver. Diabody-drug conjugates can have potent antitumor activity at well-tolerated doses and warrant further optimization for cancer therapy. [Mol Cancer Ther 2008;7(8):2486–97]

Development of orally active inhibitors of protein and cellular fucosylation

The key role played by fucose in glycoprotein and cellular function has prompted significant research toward identifying recombinant and biochemical strategies for blocking its incorporation into proteins and membrane structures. Technologies surrounding engineered cell lines have evolved for the inhibition of in vitro fucosylation, but they are not applicable for in vivo use and drug development. To address this, we screened a panel of fucose analogues and identified 2-fluorofucose and 5-alkynylfucose derivatives that depleted cells of GDP-fucose, the substrate used by fucosyltransferases to incorporate fucose into protein and cellular glycans. The inhibitors were used in vitro to generate fucose-deficient antibodies with enhanced antibody-dependent cellular cytotoxicity activities. When given orally to mice, 2-fluorofucose inhibited fucosylation of endogenously produced antibodies, tumor xenograft membranes, and neutrophil adhesion glycans. We show that oral 2-fluorofucose treatment afforded complete protection from tumor engraftment in a syngeneic tumor vaccine model, inhibited neutrophil extravasation, and delayed the outgrowth of tumor xenografts in immune-deficient mice. The results point to several potential therapeutic applications for molecules that selectively block the endogenous generation of fucosylated glycan structures.

Metabolic engineering of monoclonal antibody carbohydrates for antibody-drug conjugation.

The role that carbohydrates play in antibody function and pharmacokinetics has made them important targets for modification. The terminal fucose of the N-linked glycan structure, which has been shown to be involved in modulation of antibody-directed cellular cytotoxicity, is a particularly interesting location for potential modification through incorporation of alternative sugar structures. A library of fucose analogues was evaluated for their ability to incorporate into antibody carbohydrates in place of the native fucose. A number of efficiently incorporated molecules were identified, demonstrating the ability of fucosyltransferase VIII to utilize a variety of non-natural sugars as substrates. Among these structures was a thiolated analogue, 6-thiofucose, which was incorporated into the antibody carbohydrate with good efficiency. This unnatural thio-sugar could then be used for conjugation using maleimide chemistry to produce antibody-drug conjugates with pronounced cytotoxic activities and improved homogeneity compared to drug attachment through hinge disulfides.

Intracellular Released Payload Influences Potency and Bystander-Killing Effects of Antibody-Drug Conjugates in Preclinical Models

Antibody-drug conjugates (ADC) comprise targeting antibodies armed with potent small-molecule payloads. ADCs demonstrate specific cell killing in clinic, but the basis of their antitumor activity is not fully understood. In this study, we investigated the degree to which payload release predicts ADC activity in vitro and in vivo ADCs were generated to target different receptors on the anaplastic large cell lymphoma line L-82, but delivered the same cytotoxic payload (monomethyl auristatin E, MMAE), and we found that the intracellular concentration of released MMAE correlated with in vitro ADC-mediated cytotoxicity independent of target expression or drug:antibody ratios. Intratumoral MMAE concentrations consistently correlated with the extent of tumor growth inhibition in tumor xenograft models. In addition, we developed a robust admixed tumor model consisting of CD30(+) and CD30(-) cancer cells to study how heterogeneity of target antigen expression, a phenomenon often observed in cancer specimens, affects the treatment response. CD30-targeting ADC delivering membrane permeable MMAE or pyrrolobenzodiazepine dimers demonstrated potent bystander killing of neighboring CD30(-) cells. In contrast, a less membrane permeable payload, MMAF, failed to mediate bystander killing in vivo, suggesting local diffusion and distribution of released payloads represents a potential mechanism of ADC-mediated bystander killing. Collectively, our findings establish that the biophysical properties and amount of released payloads are chief factors determining the overall ADC potency and bystander killing. Cancer Res; 76(9); 2710-9. ©2016 AACR.

Advancing antibody drug conjugation: from the laboratory to a clinically approved anticancer drug.

The concept of using monoclonal antibodies for delivering drugs to cancer cells has been explored for decades, with early work surrounding nonspecific targets and drugs with low potencies. These studies underscored the importance of critical parameters, such as antigen and tumor target selection, linker stability, drug potency, pharmacokinetics, and conjugation methodology, in developing effective antibody drug conjugates with acceptable safety profiles. Brentuximab vedotin represents the culmination of much research and development activities in which many of these parameters were addressed. This article provides an overview of many studies that led to the development of this highly active antibody drug conjugate.