Abstract 1753: IKS014, a HER2-targeting antibody drug conjugate incorporating novel bioconjugation and tumor-selective linker technology with improved in vivo efficacy and tolerability

HER2 has long been a target of high interest for antibody and antibody drug conjugate (ADC) therapeutics due to its well-documented overexpression in breast, gastric and lung cancer. While trastuzumab and ado-trastuzumab emtansine (T-DM1) have become an integral part of treatment paradigms for HER2-positive cancer, the more recent approvals of the fam-trastuzumab deruxtecan (DS-8201) ADC and the Fc-engineered margetuximab antibody have highlighted the potential for continued improvement over existing HER2-targeting therapies. IKS014 is a HER2-directed ADC that incorporates site-directed conjugation and tumor-selective glucuronide-trigger linker technology to reduce systemic off-target toxicity while maximizing efficient intracellular lysosomal payload release, thus holding the promise of a wider therapeutic index.

IKS014 was generated by site-specific conjugation via a proprietary beta-glucuronide linker to the microtubule agent MMAF with a drug-to-antibody ratio (DAR) 2. In vitro and in vivo activity was evaluated in HER2-positive preclinical models with varying HER2 expression levels in comparison to benchmark ADCs. Pharmacokinetics (PK) and tolerability studies were conducted in rodent and cynomolgus monkey.

IKS014 conjugation resulted in a homogeneous ADC with defined DAR and good physio-chemical properties. In vitro, IKS014 demonstrated dose dependent specific cytotoxicity against Her2-positive cell lines. In JIMT-1 breast cancer xenografts with moderate HER2-expression (HER2 IHC 2+), IKS014 causes complete regressions at a single dose of 5 mg/kg and partial regressions at 1 mg/kg, while T-DM1 results in only tumor growth inhibition even at 15 mg/kg. Anti-tumor efficacy of IKS014 in NCI-N87 (HER2 3+) gastric xenografts is comparable to DS-8201 but superior to T-DM1 at equivalent single doses ranging from 1 to 5 mg/kg. In a HER2-positive patient-derived gastric cancer model (HER2 2+), IKS014 was highly active at 5mg/kg Q2W x2, while T-DM1 was inactive at the same dosing schedule. IKS014 demonstrated stable PK with a terminal half-life of 8.7 days in rat and 4.6 days in monkey, and DAR 2 was maintained for up to 4 weeks. In cynomolgus monkeys, IKS014 was tolerated at 12 mg/kg single dose and 5 mg/kg repeat dose without ocular or lung toxicity findings.

IKS014 was highly efficacious against HER2-positive tumor xenografts in vivo, including models with moderate target expression, and compared favorably to clinically validated benchmark ADCs. This improved preclinical efficacy combined with stable PK and good tolerability profile warrants further development of this novel ADC for HER2-positive cancers.

Citation Format: Jutta Deckert, Jenny Thirlway, Yun-Hee Park, Ho Young Song, Chul-Woong Chung, Xuesong Wang, Zhenshan Zhang, Robert J. Lutz. IKS014, a HER2-targeting antibody drug conjugate incorporating novel bioconjugation and tumor-selective linker technology with improved in vivo efficacy and tolerability [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1753.

Abstract 1751: Developing a next generation antibody drug conjugate for treatment of gastrointestinal cancers

Background: Gastrointestinal cancers (GI) are among the most common cancers worldwide. Current treatment options rely on a chemotherapy backbone combined with targeted antibody therapies, but the prognosis generally remains poor. Iksuda has developed a next-generation Antibody Drug Conjugate (ADC) targeting a cancer-specific carbohydrate antigen (CanAg) which is highly overexpressed in GI cancers but has no expression on normal tissues. This CanAg-targeting ADC is comprised of a proprietary PBD prodrug coupled to an anti-CanAg antibody using PermaLink conjugation technology. The PBD prodrug is designed with a tumor-selective trigger to attenuate its potency before entry of the ADC into the target cancer cells. PermaLink conjugation chemistry ensures conjugate stability. Here, the potential of this anti-CanAg ADC was explored in preclinical models as a proof of concept for targeting GI cancers.

Methods: The ADC is composed of a PBD prodrug conjugated to an anti-CanAg antibody through PermaLink to generate conjugate with a drug to antibody ratio of approximately 2. This CanAg-targeting ADC was evaluated in vitro in the CanAg-expressing Colo205 cancer cell line. In vivo efficacy studies were conducted in SCID mice with CanAg-expressing human cancer xenografts representing colorectal (Colo 205), gastric (N87) and pancreatic cancers (BxPC3). An exploratory toxicology study was conducted in cynomolgus monkeys.

Results: The CanAg-targeting ADC showed highly potent, specific activity in the Colo205 colorectal cancer cell line with IC50 values in the pM range. A single administration of anti-CanAg ADC in CanAg-positive GI xenograft models showed significant dose-dependent antitumor activity and induced complete tumor regressions with no observable toxicity. In the gastric cancer model, the anti-CanAg ADC gave a complete response by day 36 when dosed at 0.3 mg/kg, and 1 and 3 mg/kg doses exhibited long-lasting tumor regression. In the pancreatic cancer model, administration of anti-CanAg ADC at 0.3 mg/kg induced 76% tumor growth inhibition (TGI) on day 21, and at 1 mg/kg, and 3 mg/kg showed complete tumor regression by day 40 with no subsequent tumor recurrence for the 3mg/kg dose. In colorectal cancer, the anti-CanAg ADC exhibited 58% and 98% TGI with 0.3 mg/kg and 1 mg/kg doses respectively, and with the 3 mg/kg dose regressed tumor with no recurrence. The anti-CanAg ADC was well tolerated in cynomolgus monkeys with 1 mg/kg defined as the highest non-severely toxic dose (HNSTD).

Conclusions: This anti-CanAg ADC has shown favorable preclinical anti-tumor activity in gastrointestinal cancers. Our findings support the clinical development of the anti-CanAg ADC for the treatment of GI cancers.

Citation Format: Justyna H. Mysliwy, Adam Lodge, Daniel Williamson, Jenny Thirlway, Jutta Deckert, Robert J. Lutz. Developing a next generation antibody drug conjugate for treatment of gastrointestinal cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1751.

Abstract C023: IKS01, a next generation antibody drug conjugate, shows target-dependent efficacy in a platinum-resistant tumor model with low levels of folate receptor α expression

Background: Antibody drug conjugate (ADC) technologies provide selective tumor killing with increased efficacy and less off-target toxicity than standard chemotherapy. Clinical validation of this approach for solid tumors has been limited, with trastuzumab emtansine, which targets HER2+ breast cancer, being the only ADC approved for treatment of solid tumors to date. Other ADCs targeting solid tumor indications have shown promise, including mirvetuximab soravtansine (MR), an ADC directed at folate receptor α (FRA). Clinical benefit has been observed in patients with high levels of FRA expression, demonstrating that FRA can be successfully targeted by this therapeutic modality. However, the anti-tumor activity for MR appears to be limited to patients whose tumors express the highest levels of FRA only. IKS01 is an ADC composed of an FRA-targeting antibody conjugated via Iksuda’s proprietary PermaLink conjugation technology, which provides fully stable conjugation thereby avoiding payload release during circulation, to the highly-potent FGX2-62 payload. FGX2-62 is a next generation pyrridinobenzodiazepine DNA mono-alkylator with pM potency in a range of tumor indications. IKS01 has been shown to have improved efficacy in both high and moderate/low FRA-expressing tumor xenograft models, compared to MR. We have extended these findings to show efficacy in an additional low FRA-expressing ovarian model, OVCAR3, and to confirm the FRA-dependence of IKS01 efficacy in the low FRA-expressing platinum-resistant OV-90 model. Methods: Conjugation of FGX2-62 to the FRA-binding antibody Isumab01 generated the ADC, IKS01, with a drug to antibody ratio (DAR) of approximately 2. FRA expression levels in cell lines used in mouse xenograft studies were determined by flow cytometry. IKS01 anti-tumor efficacy was evaluated in the low FRA-expressing ovarian adenocarcinoma OV-90 (platinum resistant) and OVCAR-3 human cell line-derived xenograft models. An FRA-targeting benchmark ADC with a format that has shown clinical efficacy was included for comparison. Target dependence was demonstrated in the OV-90 model by comparing the efficacy of IKS01 to an isotype control antibody conjugated to FGX2-62 (DAR of approximately 2). Results: IKS01 is highly effective in causing tumor regressions in the low FRA-expressing tumor models (OVCAR3 and platinum-resistant OV-90) in vivo, at doses that are well tolerated and where the benchmark ADC showed limited or no activity. The target-dependence of IKS01 efficacy in the OV-90 model was demonstrated by comparison to a non-targeting ADC which showed minimal efficacy compared to IKS01. Conclusions: The combination of a stable conjugation technology and a novel highly-potent DNA monoalkylator has led to the development of an ADC format that has shown target-dependent anti-tumor efficacy in a pre-clinical model of platinum-resistant ovarian cancer with low levels of FRA expression. These preclinical data suggest that IKS01 is a promising therapeutic candidate for the treatment of FRA-positive ovarian cancer, even when the level of surface expression of FRA is low.

Citation Format: Jenny Thirlway, Adam Lodge, Andria Pelava, Daniel J Williamson, Davide Carta, Majid Al Nakeeb, Justyna Mysliwy, Paul J.M. Jackson, David E Thurston, Robert J Lutz. IKS01, a next generation antibody drug conjugate, shows target-dependent efficacy in a platinum-resistant tumor model with low levels of folate receptor α expression [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C023. doi:10.1158/1535-7163.TARG-19-C023

Active site modification of the β-ketoacyl-ACP synthase FabF3 of Streptomyces coelicolor affects the fatty acid chain length of the CDA lipopeptides

Using site directed mutagenesis we altered an active site residue (Phe107) of the enzyme encoded by fabF3 (SCO3248) in the Streptomyces coelicolor gene cluster required for biosynthesis of the calcium dependent antibiotics (CDAs), successfully generating two novel CDA derivatives comprising truncated (C4) lipid side chains and confirming that fabF3 encodes a KAS-II homologue that is involved in determining CDA fatty acid chain length.

Direct site-selective covalent protein immobilization catalyzed by a phosphopantetheinyl transferase

Immobilization of proteins onto solid supports is important in the preparation of functional protein microarrays and in the development of bead-based bioassays, biosensors, and industrial biocatalysts. In order to generate the stable, functional, and homogeneous materials required for these applications, attention has focused on methods that enable the efficient and site-specific covalent immobilization of recombinant proteins onto a wide range of platforms. To this end, the phosphopantetheinyl transferase Sfp was employed to catalyze the direct immobilization of recombinant proteins bearing the small, genetically encoded ybbR tag onto surfaces functionalized with CoA. Using mass spectrometry, it was shown that the Sfp catalyzes immobilization of a model acyl carrier protein (ACP) onto CoA-derivatized PEGA resin beads through specific covalent bond formation. Luciferase (Luc) and glutathione-S-transferase (GST) ybbR-fusion proteins were similarly immobilized onto PEGA resin retaining high levels of enzyme activity. This strategy was also successfully applied for the immobilization of the ACP, as well as ybbR-Luc, -GST, and -thioredoxin fusion proteins, on hydrogel microarray slides. Overall, the Sfp-catalyzed surface ligation is mild, quantitative, and rapid, occurring in a single step without prior chemical modification of the target protein. Immobilization of the target proteins directly from a cell lysate mixture was also demonstrated.

Stereospecific Enzymatic Transformation of α-Ketoglutarate to (2S,3R)-3-Methyl Glutamate during Acidic Lipopeptide Biosynthesis

The acidic lipopeptides, including the calcium-dependent antibiotics (CDA), daptomycin, and A54145, are important macrocyclic peptide natural products produced by Streptomyces species. All three compounds contain a 3-methyl glutamate (3-MeGlu) as the penultimate C-terminal residue, which is important for bioactivity. Here, biochemical in vitro reconstitution of the 3-MeGlu biosynthetic pathway is presented, using exclusively enzymes from the CDA producer Streptomyces coelicolor. It is shown that the predicted 3-MeGlu methyltransferase GlmT and its homologues DptI from the daptomycin producer Streptomyces roseosporus and LptI from the A54145 producer Streptomyces fradiae do not methylate free glutamic acid, PCP-bound glutamate, or Glu-containing CDA in vitro. Instead, GlmT, DptI, and LptI are S-adenosyl methionine (SAM)-dependent alpha-ketoglutarate methyltransferases that catalyze the stereospecific methylation of alpha-ketoglutarate (alphaKG) leading to (3R)-3-methyl-2-oxoglutarate. Subsequent enzyme screening identified the branched chain amino acid transaminase IlvE (SCO5523) as an efficient catalyst for the transformation of (3R)-3-methyl-2-oxoglutarate into (2S,3R)-3-MeGlu. Comparison of reversed-phase HPLC retention time of dabsylated 3-MeGlu generated by the coupled enzymatic reaction with dabsylated synthetic standards confirmed complete stereocontrol during enzymatic catalysis. This stereospecific two-step conversion of alphaKG to (2S,3R)-3-MeGlu completes our understanding of the biosynthesis and incorporation of beta-methylated amino acids into the nonribosomal lipopeptides. Finally, understanding this pathway may provide new possibilities for the production of modified peptides in engineered microbes.

Stereochemical Course of Tryptophan Dehydrogenation during Biosynthesis of the Calcium-Dependent Lipopeptide Antibiotics

[reaction: see text] Hydrogen atoms are abstracted from the C2' and C3'-pro-S positions of an (S)-tryptophanyl precursor, with overall syn stereochemistry, during the biosynthesis of the C-terminal Z-2',3'-dehydrotryptophan residue of the calcium-dependent lipopeptide antibiotics (CDAs) in Streptomyces coelicolor. The absence of beta-hydroxytryptophanyl, or other possible intermediates, further suggests a direct dehydrogenation mechanism similar to that proposed for the l-tryptophan 2',3'-oxidase from Chromobacterium violaceum.

In the Bacillus stearothermophilus DnaB-DnaG complex, the activities of the two proteins are modulated by distinct but overlapping networks of residues

We demonstrate the primase activity of Bacillus stearothermophilus DnaG and show that it initiates at 3'-ATC-5' and 3'-ATT-5' sites synthesizing primers that are 22 or 23 nucleotides long. In the presence of the helicase DnaB the size distribution of primers is different, and a range of additional smaller primers are also synthesized. Nine residues from the N- and C-terminal domains of DnaB, as well as its linker region, have been reported previously to affect this interaction. In Bacillus stearothermophilus only three residues from the linker region (I119 and I125) and the N-terminal domain (Y88) of DnaB have been shown previously to have direct structural importance, and I119 and I125 mediate DnaG-induced effects on DnaB activity. The functions of the other residues (L138, T191, E192, R195, and M196) are still a mystery. Here we show that the E15A, Y88A, and E15A Y88A mutants bind DnaG but are not able to modulate primer size, whereas the R195A M196A mutant inhibited the primase activity. Therefore, four of these residues, E15 and Y88 (N-terminal domain) and R195 and M196 (C-terminal domain), mediate DnaB-induced effects on DnaG activity. Overall, the data suggest that the effects of DnaB on DnaG activity and vice versa are mediated by distinct but overlapping networks of residues.

Solution structure of the helicase-interaction domain of the primase DnaG: a model for helicase activation

The helicase-primase interaction is a critical event in DNA replication and is mediated by a putative helicase-interaction domain within the primase. The solution structure of the helicase-interaction domain of DnaG reveals that it is made up of two independent subdomains: an N-terminal six-helix module and a C-terminal two-helix module that contains the residues of the primase previously identified as important in the interaction with the helicase. We show that the two-helix module alone is sufficient for strong binding between the primase and the helicase but fails to activate the helicase; both subdomains are required for helicase activation. The six-helix module of the primase has only one close structural homolog, the N-terminal domain of the corresponding helicase. This surprising structural relationship, coupled with the differences in surface properties of the two molecules, suggests how the helicase-interaction domain may perturb the structure of the helicase and lead to activation.

DnaG interacts with a linker region that joins the N- and C-domains of DnaB and induces the formation of 3-fold symmetric rings

Loading of the replicative ring helicase onto the origin of replication (oriC) is the final outcome of a well coordinated series of events that collectively constitute a primosomal cascade. Once the ring helicase is loaded, it recruits the primase and signals the switch to the polymerization mode. The transient nature of the helicase-primase (DnaB-DnaG) interaction in the Escherichia coli system has hindered our efforts to elucidate its structure and function. Taking advantage of the stable DnaB-DnaG complex in Bacillus stearothermophilus, we have reviewed conflicting mutagenic data from other bacterial systems and shown that DnaG interacts with the flexible linker that connects the N- and C-terminal domains of DnaB. Furthermore, atomic force microscopy (AFM) imaging experiments show that binding of the primase to the helicase induces predominantly a 3-fold symmetric morphology to the hexameric ring. Overall, three DnaG molecules appear to interact with the hexameric ring helicase but a small number of complexes with two and even one DnaG molecule bound to DnaB were also detected. The structural/functional significance of these data is discussed and a speculative structural model for this complex is suggested.