DNA interstrand cross-linking and in vivo antitumor activity of the extended pyrrolo[2,1-c][1,4]benzodiazepine dimer SG2057

The HER2-directed antibody-drug conjugate DHES0815A in advanced and/or metastatic breast cancer: preclinical characterization and phase 1 trial results

Approved antibody-drug conjugates (ADCs) for HER2-positive breast cancer include trastuzumab emtansine and trastuzumab deruxtecan. To develop a differentiated HER2 ADC, we chose an antibody that does not compete with trastuzumab or pertuzumab for binding, conjugated to a reduced potency PBD (pyrrolobenzodiazepine) dimer payload. PBDs are potent cytotoxic agents that alkylate and cross-link DNA. In our study, the PBD dimer is modified to alkylate, but not cross-link DNA. This HER2 ADC, DHES0815A, demonstrates in vivo efficacy in models of HER2-positive and HER2-low cancers and is well-tolerated in cynomolgus monkey safety studies. Mechanisms of action include induction of DNA damage and apoptosis, activity in non-dividing cells, and bystander activity. A dose-escalation study (ClinicalTrials.gov: NCT03451162) in patients with HER2-positive metastatic breast cancer, with the primary objective of evaluating the safety and tolerability of DHES0815A and secondary objectives of characterizing the pharmacokinetics, objective response rate, duration of response, and formation of anti-DHES0815A antibodies, is reported herein. Despite early signs of anti-tumor activity, patients at higher doses develop persistent, non-resolvable dermal, ocular, and pulmonary toxicities, which led to early termination of the phase 1 trial.

Conjugation Site Influences Antibody-Conjugated Drug PK Assays: Case Studies for Disulfide-Linked, Self-Immolating Next-Generation Antibody Drug Conjugates

Immunoaffinity (IA) LC-MS/MS pharmacokinetic (PK) assays are widely used in the field for antibody drug conjugates (ADCs) containing peptide linkers that are enzymatically cleavable, such as MC-ValCit-PAB. Conjugate PK assay strategies for these ADCs involve cleavage with cathepsin B or papain to release and measure the antibody-conjugated drug (acDrug) concentration. However, robust acDrug PK methods for disulfide-linked self-immolating ADCs are lacking as they are a different conjugation modality. We developed acDrug PK assays for next-generation disulfide-linked ADCs involving immunoaffinity capture, chemical cleavage, and LC-MS/MS. Disulfide-linked ADCs captured from plasma were chemically reduced at basic pH to release the linker-drug, followed by self-immolation to liberate the active drug, and quantified by MRM LC-MS/MS. Herein, we detail the development and optimization of this chemical cleavage acDrug PK assay, resulting in robust accuracy and precision (±20%). The conjugation site of the linker-drug on the antibody was found to affect the kinetics of drug release. Multiple biophysical and chemical characteristics, such as tertiary structure, fractional solvent accessibility, pK_a of the conjugation site, surrounding residue's pI, and electrostatic charge, may directly impact the drug release kinetics. Similar site-specific stability has been previously reported for ADCs in vivo. The assay development and qualification data for this original assay format are presented along with its application to multiple in vitro and in vivo studies across species.

Antibody-drug conjugates (ADCs) delivering pyrrolobenzodiazepine (PBD) dimers for cancer therapy

INTRODUCTION

The rationally designed pyrrolobenzodiazepine (PBD) dimers emerged around ten years ago as a new class of drug component for antibody-drug conjugates (ADC). They produce highly cytotoxic DNA cross-links, exploiting a completely different cellular target to the auristatin and maytansinoid tubulin inhibitor classes and a different mode of DNA damage to other DNA interacting warheads such as calicheamicin.

AREAS COVERED

The properties which make the PBD dimers suitable warheads for ADCs, and the development of the two main payload structures talirine and tesirine, are discussed. The clinical experience with the twenty PBD dimer-containing ADCs to enter the clinic is reviewed, with a focus on vadastuximab talirine and rovalpituzumab tesirine, both of which were discontinued following pivotal studies, and loncastuximab tesirine and camidanlumab tesirine which are progressing towards approval.

EXPERT OPINION

Reviewing the clinical efficacy and safety data from almost forty clinical trials of PBD dimer-containing ADCs highlights the complexities and challenges of ADC early clinical development. It enables some conclusions to be made about reasons for failure and suggests strategies to optimise the future clinical development of this promising class of ADCs in a rapidly expanding field.

Pyrrolobenzodiazepine Dimers as Antibody–Drug Conjugate (ADC) Payloads

The pyrrolobenzodiazepine (PBD) ring system was first discovered in the 1960s and is found in several naturally occurring potent anti-tumour antibiotics. The mode of action of PBDs involves sequence-selective [purine–guanine–purine (PuGPu)] alkylation in the minor groove of DNA through covalent binding from guanine N2 to the PBD C11-position. Dimerization of the PBD ring system gives molecules that can cross-link DNA, which leads to a substantial increase in potency and DNA binding affinity and an extension of sequence-selectivity compared with monomers. PBD dimers feature as the cytotoxic component of numerous ADCs being evaluated in clinical trials. PBD-ADC clinical candidates loncastuximab tesirine, camidanlumab tesirine and rovalpituzumab tesirine employ a PBD N10 linkage while vadastuximab talirine uses a C2-linkage. The PBD dimer scaffold is versatile and offers many opportunities to diversify the ADC platform, with extensive research being performed worldwide to develop the next generation of PBD payload–linker molecules. The search for new PBD payload–linker molecules has mainly focused on changes in payload structure (e.g. PBD C2 modification and macrocyclisation), alternative conjugation strategies (e.g. haloacetamides, ‘click’ approaches and pyridyl disulphides), non-peptide triggers in the linker (e.g. disulphides) and non-cleavable derivatives (i.e. payload release through antibody degradation).

Pre-clinical pharmacology and mechanism of action of SG3199, the pyrrolobenzodiazepine (PBD) dimer warhead component of antibody-drug conjugate (ADC) payload tesirine

Synthetic pyrrolobenzodiazepine (PBD) dimers, where two PBD monomers are linked through their aromatic A-ring phenolic C8-positions via a flexible propyldioxy tether, are highly efficient DNA minor groove cross-linking agents with potent cytotoxicity. PBD dimer SG3199 is the released warhead component of the antibody-drug conjugate (ADC) payload tesirine (SG3249), currently being evaluated in several ADC clinical trials. SG3199 was potently cytotoxic against a panel of human solid tumour and haematological cancer cell lines with a mean GI₅₀ of 151.5 pM. Cells defective in DNA repair protein ERCC1 or homologous recombination repair showed increased sensitivity to SG3199 and the drug was only moderately susceptible to multidrug resistance mechanisms. SG3199 was highly efficient at producing DNA interstrand cross-links in naked linear plasmid DNA and dose-dependent cross-linking was observed in cells. Cross-links formed rapidly in cells and persisted over 36 hours. Following intravenous (iv) administration to rats SG3199 showed a very rapid clearance with a half life as short as 8 minutes. These combined properties of cytotoxic potency, rapid formation and persistence of DNA interstrand cross-links and very short half-life contribute to the emerging success of SG3199 as a warhead in clinical stage ADCs.

An Anti-CLL-1 Antibody-Drug Conjugate for the Treatment of Acute Myeloid Leukemia

PURPOSE

The treatment of acute myeloid leukemia (AML) has not significantly changed in 40 years. Cytarabine- and anthracycline-based chemotherapy induction regimens (7 + 3) remain the standard of care, and most patients have poor long-term survival. The reapproval of Mylotarg, an anti-CD33-calicheamicin antibody-drug conjugate (ADC), has demonstrated ADCs as a clinically validated option to enhance the effectiveness of induction therapy. We are interested in developing a next-generation ADC for AML to improve upon the initial success of Mylotarg.

EXPERIMENTAL DESIGN

The expression pattern of CLL-1 and its hematopoietic potential were investigated. A novel anti-CLL-1-ADC, with a highly potent pyrrolobenzodiazepine (PBD) dimer conjugated through a self-immolative disulfide linker, was developed. The efficacy and safety profiles of this ADC were evaluated in mouse xenograft models and in cynomolgus monkeys.

RESULTS

We demonstrate that CLL-1 shares similar prevalence and trafficking properties that make CD33 an excellent ADC target for AML, but lacks expression on hematopoietic stem cells that hampers current CD33-targeted ADCs. Our anti-CLL-1-ADC is highly effective at depleting tumor cells in AML xenograft models and lacks target independent toxicities at doses that depleted target monocytes and neutrophils in cynomolgus monkeys.

CONCLUSIONS

Collectively, our data suggest that an anti-CLL-1-ADC has the potential to become an effective and safer treatment for AML in humans, by reducing and allowing for faster recovery from initial cytopenias than the current generation of ADCs for AML.

Pyrrolobenzodiazepine Dimer Antibody-Drug Conjugates: Synthesis and Evaluation of Noncleavable Drug-Linkers

Three rationally designed pyrrolobenzodiazepine (PBD) drug-linkers have been synthesized via intermediate 19 for use in antibody-drug conjugates (ADCs). They lack a cleavable trigger in the linker and consist of a maleimide for cysteine antibody conjugation, a hydrophilic spacer, and either an alkyne (6), triazole (7), or piperazine (8) link to the PBD. In vitro IC₅₀ values were 11-48 ng/mL in HER2 3+ SK-BR-3 and KPL-4 (7 inactive) for the anti-HER2 ADCs (HER2 0 MCF7, all inactive) and 0.10-1.73 μg/mL (7 inactive) in CD22 3+ BJAB and WSU-DLCL2 for anti-CD22 ADCs (CD22 0 Jurkat, all inactive at low doses). In vivo antitumor efficacy for the anti-HER2 ADCs in Founder 5 was observed with tumor stasis at 0.5-1 mg/kg, 1 mg/kg, and 3-6 mg/kg for 6, 8, and 7, respectively. Tumor stasis at 2 mg/kg was observed for anti-CD22 6 in WSU-DLCL2. In summary, noncleavable PBD-ADCs exhibit potent activity, particularly in HER2 models.

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.

From Anthramycin to Pyrrolobenzodiazepine (PBD)-Containing Antibody-Drug Conjugates (ADCs)

The pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) are a family of sequence-selective DNA minor-groove binding agents that form a covalent aminal bond between their C11-position and the C2-NH2 groups of guanine bases. The first example of a PBD monomer, the natural product anthramycin, was discovered in the 1960s, and the best known PBD dimer, SJG-136 (also known as SG2000, NSC 694501 or BN2629), was synthesized in the 1990s and has recently completed Phase II clinical trials in patients with leukaemia and ovarian cancer. More recently, PBD dimer analogues are being attached to tumor-targeting antibodies to create antibody-drug conjugates (ADCs), a number of which are now in clinical trials, with many others in pre-clinical development. This Review maps the development from anthramycin to the first PBD dimers, and then to PBD-containing ADCs, and explores both structure-activity relationships (SARs) and the biology of PBDs, and the strategies for their use as payloads for ADCs.

An Update on the Synthesis of Pyrrolo[1,4]benzodiazepines

Pyrrolo[1,4]benzodiazepines are tricyclic compounds that are considered “privileged structures” since they possess a wide range of biological activities. The first encounter with these molecules was the isolation of anthramycin from cultures of Streptomyces, followed by determination of the X-ray crystal structure of the molecule and a study of its interaction with DNA. This opened up an intensive synthetic and biological study of the pyrrolo[2,1-c][1,4]benzodiazepines that has culminated in the development of the dimer SJG-136, at present in Phase II clinical trials. The synthetic efforts have brought to light some new synthetic methodology, while the contemporary work is focused on building trimeric pyrrolo[2,1-c][1,4]benzodiazepines linked together by various heterocyclic and aliphatic chains. It is the broad spectrum of biological activities of pyrrolo[1,2-a][1,4]benzodiazepines that has maintained the interest of researchers to date whereas several derivatives of the even less studied pyrrolo[1,2-d][1,4]benzodiazepines were found to be potent non-nucleoside HIV-1 reverse transcriptase inhibitors. The present review is an update on the synthesis of pyrrolo[2,1-c][1,4]benzodiazepines since the last major review of 2011, while the overview of the synthesis of the other two tricyclic isomers is comprehensive.

Progress and Challenges in Developing Aptamer-Functionalized Targeted Drug Delivery Systems

Aptamers, which can be screened via systematic evolution of ligands by exponential enrichment (SELEX), are superior ligands for molecular recognition due to their high selectivity and affinity. The interest in the use of aptamers as ligands for targeted drug delivery has been increasing due to their unique advantages. Based on their different compositions and preparation methods, aptamer-functionalized targeted drug delivery systems can be divided into two main categories: aptamer-small molecule conjugated systems and aptamer-nanomaterial conjugated systems. In this review, we not only summarize recent progress in aptamer selection and the application of aptamers in these targeted drug delivery systems but also discuss the advantages, challenges and new perspectives associated with these delivery systems.

Current ADC Linker Chemistry

The list of ADCs in the clinic continues to grow, bolstered by the success of first two marketed ADCs: ADCETRIS® and Kadcyla®. Currently, there are 40 ADCs in various phases of clinical development. However, only 34 of these have published their structures. Of the 34 disclosed structures, 24 of them use a linkage to the thiol of cysteines on the monoclonal antibody. The remaining 10 candidates utilize chemistry to surface lysines of the antibody. Due to the inherent heterogeneity of conjugation to the multiple lysines or cysteines found in mAbs, significant research efforts are now being directed toward the production of discrete, homogeneous ADC products, via site-specific conjugation. These site-specific conjugations may involve genetic engineering of the mAb to introduce discrete, available cysteines or non-natural amino acids with an orthogonally-reactive functional group handle such as an aldehyde, ketone, azido, or alkynyl tag. These site-specific approaches not only increase the homogeneity of ADCs but also enable novel bio-orthogonal chemistries that utilize reactive moieties other than thiol or amine. This broadens the diversity of linkers that can be utilized which will lead to better linker design in future generations of ADCs.

Effect of hairpin loop structure on reactivity, sequence preference and adduct orientation of a DNA-interactive pyrrolo[2,1-c][1,4]benzodiazepine (PBD) antitumour agent

Pyrrolobenzodiazepine (PBD) monomer GWL-78 reacts faster with DNA hairpins containing a hexaethylene glycol (HEG) loop compared to hairpins containing a TTT loop due to the greater structural flexibility of the HEG.

Antibody-targeted drugs and drug resistance--challenges and solutions

Antibody-based therapy of various human malignancies has shown efficacy in the past 30 years and is now one of the most successful and leading strategies for targeted treatment of patients harboring hematological malignancies and solid tumors. Antibody-drug conjugates (ADCs) aim to take advantage of the affinity and specificity of monoclonal antibodies (mAbs) to selectively deliver potent cytotoxic drugs to antigen-expressing tumor cells. Key parameters for ADC include choosing the optimal components of the ADC (the antibody, the linker and the cytotoxic drug) and selecting the suitable cell-surface target antigen. Building on the success of recent FDA approval of brentuximab vedotin (Adcetris) and ado-trastuzumab emtansine (Kadcyla), ADCs are currently a class of drugs with a robust pipeline with clinical applications that are rapidly expanding. The more ADCs are being evaluated in preclinical models and clinical trials, the clearer are becoming the parameters and the challenges required for their therapeutic success. This rapidly growing knowledge and clinical experience are revealing novel modalities and mechanisms of resistance to ADCs, hence offering plausible solutions to such challenges. Here, we review the key parameters for designing a powerful ADC, focusing on how ADCs are addressing the challenge of multiple drug resistance (MDR) and its rational overcoming.

Intramolecular azide to alkene cycloadditions for the construction of pyrrolobenzodiazepines and azetidino-benzodiazepines

The coupling of proline- and azetidinone-substituted alkenes to 2-azidobenzoic and 2-azidobenzenesulfonic acid gives precursors that undergo intramolecular azide to alkene 1,3-dipolar cycloadditions to give imine-, triazoline- or aziridine-containing pyrrolo[1,4]benzodiazepines (PBDs), pyrrolo[1,2,5]benzothiadiazepines (PBTDs), and azetidino[1,4]benzodiazepines. The imines and aziridines are formed after loss of nitrogen from a triazoline cycloadduct. The PBDs are a potent class of antitumour antibiotics.

Antibody-drug conjugates for the treatment of cancer

With over 20 antibody-drug conjugates in clinical trials as well as a recently FDA-approved drug, it is clear that this is becoming an important and viable approach for selectively delivering highly cytotoxic agents to tumor cells while sparing normal tissue. This review discusses the critical aspects for this approach with an emphasis on the properties of the linker between the antibody and the cytotoxic payload that are required for an effective antibody-drug conjugate. Different linkers are illustrated with attention focused on (i) the specifics of attachment to the antibody, (ii) the polarity of the linker, (iii) the trigger on the linker that initiates cleavage from the drug, and (iv) the self-immolative spacer that liberates the active payload. Future directions in the field are proposed.

GC-targeted C8-linked pyrrolobenzodiazepine-biaryl conjugates with femtomolar in vitro cytotoxicity and in vivo antitumor activity in mouse models

DNA binding 4-(1-methyl-1H-pyrrol-3-yl)benzenamine (MPB) building blocks have been developed that span two DNA base pairs with a strong preference for GC-rich DNA. They have been conjugated to a pyrrolo[2,1-c][1,4]benzodiazepine (PBD) molecule to produce C8-linked PBD-MPB hybrids that can stabilize GC-rich DNA by up to 13-fold compared to AT-rich DNA. Some have subpicomolar IC50 values in human tumor cell lines and in primary chronic lymphocytic leukemia cells, while being up to 6 orders less cytotoxic in the non-tumor cell line WI38, suggesting that key DNA sequences may be relevant targets in these ultrasensitive cancer cell lines. One conjugate, 7h (KMR-28-39), which has femtomolar activity in the breast cancer cell line MDA-MB-231, has significant dose-dependent antitumor activity in MDA-MB-231 (breast) and MIA PaCa-2 (pancreatic) human tumor xenograft mouse models with insignificant toxicity at therapeutic doses. Preliminary studies suggest that 7h may sterically inhibit interaction of the transcription factor NF-κB with its cognate DNA binding sequence.