FDA Approval Summary: Fam-Trastuzumab Deruxtecan-Nxki for the Treatment of Unresectable or Metastatic HER2-Positive Breast Cancer

On December 20, 2019, the FDA granted accelerated approval to fam-trastuzumab deruxtecan-nxki [DS-8201a; T-DXd; tradename ENHERTU (Daiichi Sankyo)] for the treatment of adult patients with unresectable or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2-based regimens in the metastatic setting. Approval was based on data from study DS8201-A-U201 (DESTINY-Breast01) with supportive safety data from study DS8201-A-J101. The primary efficacy endpoint in DESTINY-Breast01 was overall response rate (ORR) based on confirmed responses by blinded independent central review (ICR) using RECIST v1.1 in all participants who were assigned to receive the recommended dose of 5.4 mg/kg while secondary endpoints included duration of response (DoR). The confirmed ORR based on ICR in these 184 patients was 60.3% [95% confidence interval (CI): 52.9-67.4] and the median DoR was 14.8 months (95% CI: 13.8-16.9). Interstitial lung disease, including pneumonitis, was experienced in patients treated with T-DXd and can be severe, life threatening, or fatal. In addition, neutropenia and left ventricular dysfunction were included as Warnings and Precautions in labeling. Other important common adverse reactions were nausea, fatigue, vomiting, alopecia, constipation, decreased appetite, anemia, diarrhea, and thrombocytopenia. Overall, the totality of efficacy and safety data supported the accelerated approval of T-DXd for the intended indication.

Rational tuning of the thiolate donor in model complexes of superoxide reductase: direct evidence for a trans influence in Fe(III)-OOR complexes

Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))Fe(II) complexes that react with tert-butyl hydroperoxide ((t)BuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)Fe(III)-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe(II) complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe(III)-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe(III)-OOR unit comes from resonance Raman (RR) spectroscopy, which shows nu(Fe-O) modes between 600-635 cm(-1) and nu(O-O) bands near 800 cm(-1). These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe(III)-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.

Influence of the nitrogen donors on nonheme iron models of superoxide reductase: high-spin Fe(III)-OOR complexes

A new five-coordinate, (N(4)S(thiolate))Fe(II) complex, containing tertiary amine donors, [Fe(II)(Me(4)[15]aneN(4))(SPh)]BPh(4) (2), was synthesized and structurally characterized as a model of the reduced active site of superoxide reductase (SOR). Reaction of 2 with tert-butyl hydroperoxide (tBuOOH) at -78 degrees C led to the generation of the alkylperoxo-iron(III) complex [Fe(III)(Me(4)[15]aneN(4))(SPh)(OOtBu)](+) (2a). The nonthiolate-ligated complex, [Fe(II)(Me(4)[15]aneN(4))(OTf)(2)] (3), was also reacted with tBuOOH and yielded the corresponding alkylperoxo complex [Fe(III)(Me(4)[15]aneN(4))(OTf)(OOtBu)](+) (3a) at an elevated temperature of -23 degrees C. These species were characterized by low-temperature UV-vis, EPR, and resonance Raman spectroscopies. Complexes 2a and 3a exhibit distinctly different spectroscopic signatures than the analogous alkylperoxo complexes [Fe(III)([15]aneN(4))(SAr)(OOR)](+), which contain secondary amine donors. Importantly, alkylation at nitrogen leads to a change from low-spin (S = 1/2) to high-spin (S = 5/2) of the iron(III) center. The resonance Raman data reveal that this change in spin state has a large effect on the nu(Fe-O) and nu(O-O) vibrations, and a comparison between 2a and the nonthiolate-ligated complex 3a shows that axial ligation has an additional significant impact on these vibrations. To our knowledge this study is the first in which the influence of a ligand trans to a peroxo moiety has been evaluated for a structurally equivalent pair of high-spin/low-spin peroxo-iron(III) complexes. The implications of spin state and thiolate ligation are discussed with regard to the functioning of SOR.

A Low-Spin Alkylperoxo−Iron(III) Complex with Weak Fe−O and O−O Bonds: Implications for the Mechanism of Superoxide Reductase

The synthesis of a mononuclear, five-coordinate ferrous complex [([15]aneN4)FeII(SPh)](BF4) (1) is reported. This complex is a new model of the reduced active site of the enzyme superoxide reductase (SOR), which is comprised of a [(NHis)4(Scys)FeII] center. Complex 1 reacts with alkylhydroperoxides (tBuOOH, cumenylOOH) at low temperature to give a metastable, dark red intermediate (2a: R = tBu; 2b: R = cumenyl) that has been characterized by UV-vis, EPR, and resonance Raman spectroscopy. The UV-vis spectrum (-80 degrees C) reveals a 526 nm absorbance (epsilon = 2150 M-1 cm-1) for 2a and a 527 nm absorbance (epsilon = 1650 M-1 cm-1) for 2b, indicative of alkylperoxo-to-iron(III) LMCT transitions, and the EPR data (77 K) show that both intermediates are low-spin iron(III) complexes (g = 2.20 and 1.97). Definitive identification of the Fe(III)-OOR species comes from RR spectra, which give nu(Fe-O) = 612 (2a) and 615 (2b) cm-1, and nu(O-O) = 803 (2a) and 795 (2b) cm-1. The assignments for 2a were confirmed by 18O substitution (tBu18O18OH), resulting in a 28 cm-1 downshift for nu(Fe-18O), and a 46 cm-1 downshift for nu(18O-18O). These data show that 2a and 2b are low-spin FeIII-OOR species with weak Fe-O bonds and suggest that a low-spin intermediate may occur in SOR, as opposed to previous proposals invoking high-spin intermediates.

X-ray absorption spectroscopy and reactivity of thiolate-ligated Fe(III)-OOR complexes

The reaction of a series of thiolate-ligated iron(II) complexes [Fe(II)([15]aneN(4))(SC(6)H(5))]BF(4) (1), [Fe(II)([15]aneN(4))(SC(6)H(4)-p-Cl)]BF(4) (2), and [Fe(II)([15]aneN(4))(SC(6)H(4)-p-NO(2))]BF(4) (3) with alkylhydroperoxides at low temperature (-78 °C or -40 °C) leads to the metastable alkylperoxo-iron(III) species [Fe(III)([15]aneN(4))(SC(6)H(5))(OOtBu)]BF(4) (1a), [Fe(III)([15]aneN(4))(SC(6)H(4)-p-Cl)(OOtBu)]BF(4) (2a), and [Fe(III)([15]aneN(4))(SC(6)H(4)-p-NO(2))(OOtBu)]BF(4) (3a), respectively. X-ray absorption spectroscopy (XAS) studies were conducted on the Fe(III)-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (∼7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the Fe(III)-OOR species range from 7121-7125 eV and correlate with the nature of the thiolate donor. Extended X-ray absorption fine structure (EXAFS) analysis of the iron(II) complexes 1-3 in CH(2)Cl(2) show that their solid state structures remain intact in solution. The EXAFS data on 1a-3a confirm their proposed structures as mononuclear, 6-coordinate Fe(III)-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a-3a are 1.82-1.85 Å, significantly longer than other low-spin Fe(III)-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a- 3a show an important influence of the thiolate donor.

Phosphate Triester Hydrolysis Promoted by an N2S(thiolate)Zn Complex: Mechanistic Implications for the Metal-Dependent Reactivity of Peptide Deformylase

The zinc(II) complex (PATH)ZnOH, where PATH is an N2S(thiolate) ligand, has been investigated for its ability to promote the hydrolysis of the phosphate triester tris(4-nitrophenyl) phosphate (TNP). The hydrolysis of TNP was examined as a function of PATH-zinc(II) complex concentration, substrate concentration, and pH in a water/ethanol mixture (66:33 v/v) at 25 degrees C. The reaction is first order in both zinc(II) complex and substrate, and the second-order rate constants were derived from linear plots of the observed pseudo-first-order rate constants versus zinc complex concentration at different pH values. A pH-rate profile yielded a kinetic pK(a) of 8.52(5) for the zinc-bound water molecule and a pH-independent rate constant of 16.1(7) M(-1) s(-1). Temperature-dependent studies showed linear Eyring behavior, yielding the activation parameters DeltaH++ = 36.9(1) kJ mol(-1) and DeltaS++ = -106.7(4) J mol(-1) K(-1). Interpretation of the kinetic data leads to the conclusion that hydrolysis of TNP takes place through a hybrid mechanism, in which the metal center plays a dual role of providing a nucleophilic hydroxide and activating the substrate through a Lewis acid effect. The synthesis and structural characterization of the related nickel(II) and iron(II) complexes [(PATH)2Ni2]Br2 (2) and (PATH)2Fe2Cl2 (3) are also described. Taken together, these data suggest a possible explanation for the low reactivity of the zinc(II) form of peptide deformylase as compared to the iron(II) form.

Secondary interactions involving zinc-bound ligands: roles in structural stabilization and macromolecular interactions

A large number of proteins contain bound zinc ions. These zinc ions are frequently coordinated by a combination of histidine and cysteine residues. In addition to atoms that coordinate directly to the zinc ions, these side chains have groups that can donate or accept hydrogen bonds from other groups. These secondary interactions can help stabilize the zinc-binding sites, can contribute to protein folding and stability, and, on occasion, can participate in interactions with other macromolecules. Five examples of these secondary interactions are discussed: carbonic anhydrase (where secondary interactions involving histidine residues stabilize the zinc-binding site thermodynamically and kinetically), retroviral nucleocapsid proteins and TRAF proteins (where cysteinate sulfur to peptide NH hydrogen bonds contribute to the structural relationships between adjacent domains), and nucleic acid binding proteins, Zif268 and TIS11 where secondary interactions participate in protein-nucleic acid interactions.

Synthesis, structure, and characterization of molybdenum(VI) imido complexes with N-salicylidene-2-aminophenol

Diimido complexes of the type Mo(NAr)2Cl2(dme) (dme = 1,2-dimethoxyethane) react with N-salicylidene-2-aminophenol (sapH2) in methanol in the presence of 2 equiv of triethylamine to form complexes with the general formula Mo(NAr)(1,2-OC6H4NH)(sap). The structures of three of these compounds (NAr = 2,6-dimethylphenylimido (1), 2,4,6-trimethylphenylimido (2), 2-tert-butylphenylimido3) have been determined by X-ray crystallography. The coordination sphere around the Mo is a distorted octahedron. The oxygen from the 2-aminophenol is trans to the imido nitrogen, whereas the amido nitrogen and the tridentate sap occupy the four equatorial positions. The Mo-N-C imido linkages have angles of 167.5(2) degrees (1), 163.2(2) degrees (2), and 162.4(1) degrees (3). A precursor complex to the imido-amido complex, Mo(NAr)(sap)(OCH3)2 (4, NAr = 2,4,6-trimethylphenylimido), has been isolated and characterized. Compound 4 reacts with 2-aminophenol to form 2, with 2-aminothiophenol to form Mo(NAr)(1,2-SC6H4NH)(sap) (5), with catechol to form Mo(NAr)(1,2-OC6H4O)(sap) (6), with naphthalene-2,3-diol to form Mo(NAr)(naphthalene-2,3-diolate)(sap) (7), with 1,2-benzenedithiol to form Mo(NAr)(1,2-SC6H4S)(sap) (8), and with 1,2-phenylenediamine to form Mo(NAr)(1,2-HNC6H4NH)(sap) (9). The structures of compounds 5-9 have been determined by X-ray crystallography. With the exception of compound 8, the structures are similar to those of 1,2, and 3, with the bidentate ligand occupying one axial and one equatorial position. In 8, 1,2-benzendithiolate occupies two equatorial positions, and the nitrogen from sap is located trans to the imido nitrogen. All complexes were characterized by 1H NMR spectroscopy, cyclic voltammetry, and UV-vis spectroscopy. When a solution of 4 is exposed to moisture-containing air, MoO2(sap)(CH3OH) (10) is formed. The structure of 10 was also determined.

Revisiting and re-engineering the classical zinc finger peptide: consensus peptide-1 (CP-1)

Zinc plays key structural and catalytic roles in biology. Structural zinc sites are often referred to as zinc finger (ZF) sites, and the classical ZF contains a Cys2His2 motif that is involved in coordinating Zn(II). An optimized Cys2His2 ZF, named consensus peptide 1 (CP-1), was identified more than 20 years ago using a limited set of sequenced proteins. We have reexamined the CP-1 sequence, using our current, much larger database of sequenced proteins that have been identified from high-throughput sequencing methods, and found the sequence to be largely unchanged. The CCHH ligand set of CP-1 was then altered to a CAHH motif to impart hydrolytic activity. This ligand set mimics the His2Cys ligand set of peptide deformylase (PDF), a hydrolytically active M(II)-centered (M = Zn or Fe) protein. The resultant peptide [CP-1(CAHH)] was evaluated for its ability to coordinate Zn(II) and Co(II) ions, adopt secondary structure, and promote hydrolysis. CP-1(CAHH) was found to coordinate Co(II) and Zn(II) and a pentacoordinate geometry for Co(II)-CP-1(CAHH) was implicated from UV-vis data. This suggests a His2Cys(H2O)2 environment at the metal center. The Zn(II)-bound CP-1(CAHH) was shown to adopt partial secondary structure by 1-D (1)H NMR spectroscopy. Both Zn(II)-CP-1(CAHH) and Co(II)-CP-1(CAHH) show good hydrolytic activity toward the test substrate 4-nitrophenyl acetate, exhibiting faster rates than most active synthetic Zn(II) complexes.

A combinatorial approach to minimal peptide models of a metalloprotein active site

Screening of a "one-bead-one-compound" peptide library containing biomimetic His/Cys ligands has led to the discovery of sequences that hydrolyze ester substrates in combination with Zn2+.

Analytical characterization of aberrant trisulfide bond formation in therapeutic proteins and their impact on product quality

Post translational modifications (PTMs) of proteins play an integral role in maintaining the overall structure and function of proteins including their proper folding, binding, and potency. However, not all PTMs play a positive role in protein drugs as some can lead to product-related impurities that negatively impact protein function. One example of a PTM is trisulfide formation, which appears as a product related species in multiple biologic drug products. The impacts of trisulfide formation on protein structure, stability, potency, and safety remains under investigation. Herein, we investigated and report the impact of aberrant trisulfides on erythropoietin (EPO) and somatropin (growth hormone/GH) therapeutic proteins. Utilizing LC-MS we show that one EPO product contains measurable basal levels of trisulfide bonds in its formulation and exposure to H2S induced aberrant trisulfides in all products investigated. We report that exposure to H2S produces moderate effects on protein stability via thermal melting monitored by circular dichroism, protein purity utilizing size exclusion chromatography, and particle content using micro-flow imaging. No changes were observed in protein folding via circular dichroism, immunogenicity screening via a THP1-blue assay, or receptor binding activity via biolayer interferometry. Together, these data provide evidence on the effects of aberrant trisulfide formation on overall product quality.