An Additional Spirocyclization for Duocarmycin SA

Base-Promoted Intramolecular Addition of Vinyl Cyclopropanecarboxamides to Access Conformationally Restricted Aza[3.1.0]bicycles

3-Azabicyclo[3.1.0]hexanes are common structural components in natural products and bioactive compounds. Traditionally, the metal-mediated cyclopropanation domino reaction of chain enzymes is the most commonly used strategy for the construction of this type of aza[3.1.0]bicycle derivative. In this study, a base-promoted intramolecular addition of alkenes used to deliver conformationally restricted highly substituted aza[3.1.0]bicycles is reported. This reaction was tailor-made for saturated aza[3.1.0] bicycle-containing fused bicyclic compounds that may be applied in the development of concise and divergent total syntheses of bioactive compounds.

Probing cytochrome P450 (CYP) bioactivation with chloromethylindoline bioprecursors derived from the duocarmycin family of compounds

The duocarmycins belong to a class of agent which has great potential for use in cancer therapy. Their exquisite potency means they are too toxic for systemic use, and targeted approaches are required to unlock their clinical potential. In this study, we have explored seco-OH-chloromethylindoline (CI) duocarmycin-based bioprecursors for their potential for cytochrome P450 (CYP)-mediated cancer cell kill. We report on synthetic and biological explorations of racemic seco-CI-MI, where MI is a 5-methoxy indole motif, and dehydroxylated analogues. We show up to a 10-fold bioactivation of de-OH CI-MI and a fluoro bioprecursor analogue in CYP1A1-transfected cells. Using CYP bactosomes, we also demonstrate that CYP1A2 but not CYP1B1 or CYP3A4 has propensity for potentiating these compounds, indicating preference for CYP1A bioactivation.

The Difference a Single Atom Can Make: Synthesis and Design at the Chemistry-Biology Interface

A Perspective of work in our laboratory on the examination of biologically active compounds, especially natural products, is presented. In the context of individual programs and along with a summary of our work, selected cases are presented that illustrate the impact single atom changes can have on the biological properties of the compounds. The examples were chosen to highlight single heavy atom changes that improve activity, rather than those that involve informative alterations that reduce or abolish activity. The examples were also chosen to illustrate that the impact of such single-atom changes can originate from steric, electronic, conformational, or H-bonding effects, from changes in functional reactivity, from fundamental intermolecular interactions with a biological target, from introduction of a new or altered functionalization site, or from features as simple as improvements in stability or physical properties. Nearly all the examples highlighted represent not only unusual instances of productive deep-seated natural product modifications and were introduced through total synthesis but are also remarkable in that they are derived from only a single heavy atom change in the structure.

Solid-Phase Synthesis of Duocarmycin Analogues and the Effect of C-Terminal Substitution on Biological Activity

The duocarmycins are potent antitumor agents with potential for use in the development of antibody-drug conjugates (ADCs) as well as being clinical candidates in their own right. In this article, we describe the synthesis of a duocarmycin monomer (DSA) that is suitably protected for utilization in solid-phase synthesis. The synthesis was performed on a large scale, and the resulting racemic protected Fmoc-DSA subunit was separated by supercritical fluid chromatography (SFC) into the single enantiomers; its application to solid-phase synthesis methodology gave a series of monomeric and extended duocarmycin analogues with amino acid substituents. The DNA sequence selectivity was similar to that in previous reports for both the monomeric and extended compounds. Substitution at the C-terminus of duocarmycin caused a decrease in antiproliferative activity for all of the compounds studied. An extended compound containing an alanine at the C-terminus was converted to the primary amide or to an extended structure containing a terminal tertiary amine, but this had no beneficial effects on biological activity.

In vivo biotransformations of antibody–drug conjugates

The selective delivery of potent pharmacologically active compounds to target tissue or cells by antibody–drug conjugates makes this immuno-conjugate a promising modality for the treatment of cancers. A thorough understanding of the structural integrity of the linker, the payload and the conjugation site during biological exposure is critical throughout the process of novel linker-payload design and optimization of PK profile. This understanding is a key aspect of the effort to maximize efficacy while minimizing toxicity in preclinical testing and to ensure the translation to the clinical setting. The complexity of this bioconjugate modality is a source of significant challenge for analytical interrogation and analysis in vivo. Therefore, we report herein a survey of various types of biotransformation events that have been elucidated in recent years.

Evaluation of a reductively activated duocarmycin prodrug against murine and human solid cancers

In treating cancer with clinically approved chemotherapies, the high systemic toxicity and lack of selectivity for malignant cells often result in an overall poor response rate. One pharmacological approach to improve patient response is to design targeted therapies that exploit the cancer milieu by reductively activating prodrugs, which results in the selective release of the free drug in the tumor tissue. Previously, we characterized prodrugs of seco-CBI-indole 2 (CBI-indole 2) designed to be activated in hypoxic tumor microenvironments, wherein the tumor maintains higher concentrations of "reducing" nucleophiles capable of preferentially releasing the free drug by nucleophilic attack on a weak N-O bond. Of these prodrugs, BocNHO-CBI-indole 2 (BocNHO) surpassed the efficacy of the free drug, CBI-indole 2, when examined in vivo in the murine L1210 leukemia model and demonstrated reduced toxicity suggesting a targeted or sustained release in vivo. Herein, we further examine the biological activity of the BocNHO prodrug in murine breast cancer, as well as human prostate and lung cancer cell lines, in vitro. Notably, BocNHO manifests potent antiproliferative and cytotoxic activity in all three tumor cell lines. However, in comparison to the activity observed in the murine cancer cell line, the human cancer cell lines were less sensitive, especially at early timepoints for cytotoxicity. Based on these findings, BocNHO was tested in a more clinically relevant orthotopic lung tumor model, revealing significant efficacy and reduced toxicity compared with the free drug. The data suggests that this pharmacological approach to designing targeted therapies is amenable to human solid tumors.

Palladium(0)-catalyzed cyclopropanation of benzyl bromides via C(sp3)–H bond activation

A novel Pd(0)-catalyzed domino cyclopropanation reaction was established. The process involves a Heck-type coupling reaction and a C(sp³)–H activation.

Synthesis and evaluation of a series of C5'-substituted duocarmycin SA analogs

The synthesis and evaluation of a key series of analogs of duocarmycin SA, bearing a single substituent at the C5' position of the DNA binding subunit, are described.

Synthesis and evaluation of duocarmycin SA analogs incorporating the methyl 1,2,8,8a-tetrahydrocyclopropa[c]oxazolo[2,3-e]indol-4-one-6-carboxylate (COI) alkylation subunit

The design, synthesis and evaluation of methyl 1,2,8,8a-tetrahydrocyclopropa[c]oxazolo[2,3-e]indol-4-one-6-carboxylate (COI) derivatives are detailed representing analogs of duocarmycin SA containing an oxazole replacement for the fused pyrrole found in the alkylation subunit.

Synthesis and evaluation of a thio analogue of duocarmycin SA

The design, synthesis, and preliminary evaluation of methyl 1,2,8,8a-tetrahydrocyclopropa[c]thieno[3,2-e]indol-4-one-6-carboxylate (CTI) derivatives are detailed representing a single atom change (N to S) embedded in the duocarmycin SA alkylation subunit.

Synthesis and Preliminary Evaluation of Duocarmycin Analogues Incorporating the 1,2,11,11a-Tetrahydrocyclopropa[c]naphtho[2,3-e]indol-4-one (CNI) and 1,2,11,11a-Tetrahydrocyclopropa[c]naphtho[1,2-e]indol-4-one (iso-CNI) Alkylation Subunits

Efficient syntheses and a preliminary evaluation of 1,2,11,11a-tetrahydrocyclopropa[c]-naphtho[2,3-e]indole (CNI) and 1,2,11,11a-tetrahydrocyclopropa[c]naphtho[1,2-e]indole (iso-CNI), and their derivatives containing an anthracene and phenanthrene variant of the CC-1065 or duocarmycin alkylation subunit are detailed.

Total synthesis and evaluation of iso-duocarmycin SA and iso-yatakemycin

The total synthesis and evaluation of iso-duocarmycin SA (5) and iso-yatakemycin (6), representing key analogues of the corresponding natural products incorporating an isomeric alkylation subunit, are detailed. This pyrrole isomer of the natural alkylation subunit displayed an enhanced reaction regioselectivity and a 2-fold diminished stability. Although still exceptionally potent, the iso-duocarmycin SA derivatives and natural product analogues exhibited a corresponding approximate 3-5-fold reduction in cytotoxic activity [L1210 IC(50) for (+)-iso-duocarmycin SA = 50 pM and for (+)-iso-yatakemycin = 15 pM] consistent with their placement on a parabolic relationship correlating activity with reactivity. The DNA alkylation selectivity of the resulting key natural product analogues was unaltered by the structure modification in spite of the minor-groove presentation of a potential H-bond donor. Additionally, a unique ortho-spirocyclization with such derivatives was explored via the preparation, characterization, and evaluation of 34 that is incapable of the more conventional para-spirocyclization. Although 34 proved sufficiently stable for isolation and characterization, it displayed little stability in protic solvents (t(1/2) = 0.19 h at pH 3, t(1/2) = 0.20 h at pH 7), a pH-independent (H(+) independent) solvolysis rate profile at pH 3/4-7, and a much reduced cytotoxic potency, but a DNA alkylation selectivity and efficiency comparable to those of duocarmycin SA and iso-duocarmycin SA. The implications of these observations on the source of the DNA alkylation selectivity and catalysis for this class of natural products are discussed.