Mechanistic studies of an unprecedented enzyme-catalysed 1,2-phosphono-migration reaction.

(S)-2-hydroxypropylphosphonate ((S)-2-HPP) epoxidase (HppE) is a mononuclear non-haem-iron-dependent enzyme responsible for the final step in the biosynthesis of the clinically useful antibiotic fosfomycin. Enzymes of this class typically catalyse oxygenation reactions that proceed via the formation of substrate radical intermediates. By contrast, HppE catalyses an unusual dehydrogenation reaction while converting the secondary alcohol of (S)-2-HPP to the epoxide ring of fosfomycin. Here we show that HppE also catalyses a biologically unprecedented 1,2-phosphono migration with the alternative substrate (R)-1-HPP. This transformation probably involves an intermediary carbocation, based on observations with additional substrate analogues, such as (1R)-1-hydroxyl-2-aminopropylphosphonate, and model reactions for both radical- and carbocation-mediated migration. The ability of HppE to catalyse distinct reactions depending on the regio- and stereochemical properties of the substrate is given a structural basis using X-ray crystallography. These results provide compelling evidence for the formation of a substrate-derived cation intermediate in the catalytic cycle of a mononuclear non-haem-iron-dependent enzyme. The underlying chemistry of this unusual phosphono migration may represent a new paradigm for the in vivo construction of phosphonate-containing natural products that can be exploited for the preparation of new phosphonate derivatives.

Purification and characterization of the epoxidase catalyzing the formation of fosfomycin from Pseudomonas syringae.

The final step in the biosynthesis of fosfomycin in Streptomyces wedmorensis is catalyzed by ( S)-2-hydroxypropylphosphonic acid (HPP) epoxidase ( Sw-HppE). A homologous enzyme from Pseudomonas syringae whose encoding gene ( orf3) shares a relatively low degree of sequence homology with the corresponding Sw-HppE gene has recently been isolated. This purified P. syringae protein was determined to catalyze the epoxidation of ( S)-HPP to fosfomycin and the oxidation of ( R)-HPP to 2-oxopropylphosphonic acid under the same conditions as Sw-HppE. Therefore, this protein is indeed a true HPP epoxidase and is termed Ps-HppE. Like Sw-HppE, Ps-HppE was determined to be post-translationally modified by the hydroxylation of a putative active site tyrosine (Tyr95). Analysis of the Fe(II) center by EPR spectroscopy using NO as a spin probe and molecular oxygen surrogate reveals that Ps-HppE's metal center is similar, but not identical, to that of Sw-HppE. The identity of the rate-determining step for the ( S)-HPP and ( R)-HPP reactions was determined by measuring primary deuterium kinetic effects, and the outcome of these results was correlated with density functional theory calculations. Interestingly, the reaction using the nonphysiological substrate ( R)-HPP was 1.9 times faster than that with ( S)-HPP for both Ps-HppE and Sw-HppE. This is likely due to the difference in bond dissociation energy of the abstracted hydrogen atom for each respective reaction. Thus, despite the low level of amino acid sequence identity, Ps-HppE is a close mimic of Sw-HppE, representing a second example of a non-heme iron-dependent enzyme capable of catalyzing dehydrogenation of a secondary alcohol to form a new C-O bond.

Modulation of Macropinocytosis-Mediated Internalization Decreases Ocular Toxicity of Antibody-Drug Conjugates.

AGS-16C3F is an antibody-drug conjugate (ADC) against ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3) containing the mcMMAF linker-payload currently in development for treatment of metastatic renal cell carcinoma. AGS-16C3F and other ADCs have been reported to cause ocular toxicity in patients by unknown mechanisms. To investigate this toxicity, we developed an in vitro assay using human corneal epithelial cells (HCEC) and show that HCECs internalized AGS-16C3F and other ADCs by macropinocytosis, causing inhibition of cell proliferation. We observed the same mechanism for target-independent internalization of AGS-16C3F in fibroblasts and human umbilical vein endothelial cells (HUVEC). Macropinocytosis-mediated intake of macromolecules is facilitated by the presence of positive charges or hydrophobic residues on the surface of the macromolecule. Modification of AGS-16C3F, either by attachment of poly-glutamate peptides, mutation of residue K16 to D on AGS-16C3F [AGS-16C3F(K16D)], or decreasing the overall hydrophobicity via attachment of polyethylene glycol moieties, significantly reduced cytotoxicity against HCECs and other primary cells. Rabbits treated with AGS-16C3F showed significant ocular toxicity, whereas those treated with AGS-16C3F(K16D) presented with less severe and delayed toxicities. Both molecules displayed similar antitumor activity in a mouse xenograft model. These findings establish a mechanism of action for target-independent toxicities of AGS-16C3F and ADCs in general, and provide methods to ameliorate these toxicities.Significance: These findings reveal a mechanism for nonreceptor-mediated toxicities of antibody drug conjugates and potential solutions to alleviate these toxicities. Cancer Res; 78(8); 2115-26. ©2018 AACR.

AGS67E, an Anti-CD37 Monomethyl Auristatin E Antibody-Drug Conjugate as a Potential Therapeutic for B/T-Cell Malignancies and AML: A New Role for CD37 in AML.

CD37 is a tetraspanin expressed on malignant B cells. Recently, CD37 has gained interest as a therapeutic target. We developed AGS67E, an antibody-drug conjugate that targets CD37 for the potential treatment of B/T-cell malignancies. It is a fully human monoclonal IgG2 antibody (AGS67C) conjugated, via a protease-cleavable linker, to the microtubule-disrupting agent monomethyl auristatin E (MMAE). AGS67E induces potent cytotoxicity, apoptosis, and cell-cycle alterations in many non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL) cell lines and patient-derived samples in vitro. It also shows potent antitumor activity in NHL and CLL xenografts, including Rituxan-refractory models. During profiling studies to confirm the reported expression of CD37 in normal tissues and B-cell malignancies, we made the novel discovery that the CD37 protein was expressed in T-cell lymphomas and in AML. AGS67E bound to >80% of NHL and T-cell lymphomas, 100% of CLL and 100% of AML patient-derived samples, including CD34(+)CD38(-) leukemic stem cells. It also induced cytotoxicity, apoptosis, and cell-cycle alterations in AML cell lines and antitumor efficacy in orthotopic AML xenografts. Taken together, this study shows not only that AGS67E may serve as a potential therapeutic for B/T-cell malignancies, but it also demonstrates, for the first time, that CD37 is well expressed and a potential drug target in AML.

In vitro and in vivo evaluation of cysteine and site specific conjugated herceptin antibody-drug conjugates.

Antibody drug conjugates (ADCs) are monoclonal antibodies designed to deliver a cytotoxic drug selectively to antigen expressing cells. Several components of an ADC including the selection of the antibody, the linker, the cytotoxic drug payload and the site of attachment used to attach the drug to the antibody are critical to the activity and development of the ADC. The cytotoxic drugs or payloads used to make ADCs are typically conjugated to the antibody through cysteine or lysine residues. This results in ADCs that have a heterogeneous number of drugs per antibody. The number of drugs per antibody commonly referred to as the drug to antibody ratio (DAR), can vary between 0 and 8 drugs for a IgG1 antibody. Antibodies with 0 drugs are ineffective and compete with the ADC for binding to the antigen expressing cells. Antibodies with 8 drugs per antibody have reduced in vivo stability, which may contribute to non target related toxicities. In these studies we incorporated a non-natural amino acid, para acetyl phenylalanine, at two unique sites within an antibody against Her2/neu. We covalently attached a cytotoxic drug to these sites to form an ADC which contains two drugs per antibody. We report the results from the first direct preclinical comparison of a site specific non-natural amino acid anti-Her2 ADC and a cysteine conjugated anti-Her2 ADC. We report that the site specific non-natural amino acid anti-Her2 ADCs have superior in vitro serum stability and preclinical toxicology profile in rats as compared to the cysteine conjugated anti-Her2 ADCs. We also demonstrate that the site specific non-natural amino acid anti-Her2 ADCs maintain their in vitro potency and in vivo efficacy against Her2 expressing human tumor cell lines. Our data suggests that site specific non-natural amino acid ADCs may have a superior therapeutic window than cysteine conjugated ADCs.

CEACAM5-targeted therapy of human colonic and pancreatic cancer xenografts with potent labetuzumab-SN-38 immunoconjugates.

PURPOSE: To improve the efficacy and reduce the gastrointestinal toxicity of the cancer prodrug, CPT-11, we have developed immunoconjugates of its active form, SN-38, and an anti-CEACAM5 antibody for targeted chemotherapy. EXPERIMENTAL DESIGN: SN-38 conjugates of the anti-CEACAM5 monoclonal antibody, labetuzumab (hMN-14), varying in the nature of the cross-linker attachment at the drug's 20-hydroxyl position, were evaluated in vitro, in metastatic and/or s.c. human colonic and pancreatic cancer xenografts in nude mice using appropriate controls, and in a CEACAM5-negative tumor model. RESULTS: A pilot study in a s.c. LS174T model of human colonic carcinoma established the relative effectiveness of different conjugates. In the lung metastatic model of GW-39 human colonic carcinoma in nude mice, therapy with two specific labetuzumab-SN-38 conjugates, using 0.25 mg SN-38 equivalent/kg, q4d x 8, significantly extended median survival time versus controls (P < 0.002). In an expanded evaluation in the s.c. LS174T xenograft model, specific SN-38 conjugates produced significant tumor growth control and increases in median survival time versus other controls, including CPT-11 at a 33-fold greater cumulative dose (P < 0.01). An improvement was also observed in the therapy of a s.c. human pancreatic tumor xenograft. In a CEACAM5-negative systemic lymphoma xenograft, one labetuzumab-SN-38 conjugate examined was ineffective, whereas the conjugate specific for the tumor model produced 100% survival. CONCLUSIONS: The promising labetuzumab-SN-38 conjugates developed showed selective therapeutic efficacy in human tumor models at nontoxic doses that were a fraction of the CPT-11 doses used.

Antibody conjugates of 7-ethyl-10-hydroxycamptothecin (SN-38) for targeted cancer chemotherapy.

CPT-11 is a clinically used cancer drug, and it is a prodrug of the potent topoisomerase I inhibitor, SN-38 (7-ethyl-10-hydroxycamptothecin). To bypass the need for the in vivo conversion of CPT-11 and increase the therapeutic index, bifunctional derivatives of SN-38 were prepared for use in antibody-based targeted therapy of cancer. The general synthetic scheme incorporated an acetylene-azide click cycloaddition step in the design, a short polyethylene glycol spacer for aqueous solubility, and a maleimide group for conjugation. Conjugates of a humanized anti-CEACAM5 monoclonal antibody, hMN-14, prepared using these SN-38 derivatives were evaluated in vitro for stability in buffer and human serum and for antigen-binding and cytotoxicity in a human colon adenocarcinoma cell line. Conjugates of hMN-14 and SN-38 derivatives 16 and 17 were found promising for further development.

Determination of the substrate binding mode to the active site iron of (S)-2-hydroxypropylphosphonic acid epoxidase using 17O-enriched substrates and substrate analogues.

(S)-2-Hydroxypropylphosphonic acid epoxidase (HppE) is an O2-dependent, nonheme Fe(II)-containing oxidase that converts (S)-2-hydroxypropylphosphonic acid ((S)-HPP) to the regio- and enantiomerically specific epoxide, fosfomycin. Use of (R)-2-hydroxypropylphosphonic acid ((R)-HPP) yields the 2-keto-adduct rather than the epoxide. Here we report the chemical synthesis of a range of HPP analogues designed to probe the basis for this specificity. In past studies, NO has been used as an O2 surrogate to provide an EPR probe of the Fe(II) environment. These studies suggest that O2 binds to the iron, and substrates bind in a single orientation that strongly perturbs the iron environment. Recently, the X-ray crystal structure showed direct binding of the substrate to the iron, but both monodentate (via the phosphonate) and chelated (via the hydroxyl and phosphonate) orientations were observed. In the current study, hyperfine broadening of the homogeneous S = 3/2 EPR spectrum of the HppE-NO-HPP complex was observed when either the hydroxyl or the phosphonate group of HPP was enriched with 17O (I = 5/2). These results indicate that both functional groups of HPP bind to Fe(II) ion at the same time as NO, suggesting that the chelated substrate binding mode dominates in solution. (R)- and (S)-analogue compounds that maintained the core structure of HPP but added bulky terminal groups were turned over to give products analogous to those from (R)- and (S)-HPP, respectively. In contrast, substrate analogues lacking either the phosphonate or hydroxyl group were not turned over. Elongation of the carbon chain between the hydroxyl and phosphonate allowed binding to the iron in a variety of orientations to give keto and diol products at positions determined by the hydroxyl substituent, but no stable epoxide was formed. These studies show the importance of the Fe(II)-substrate chelate structure to active antibiotic formation. This fixed orientation may align the substrate next to the iron-bound activated oxygen species thought to mediate hydrogen atom abstraction from the nearest substrate carbon.