Cyclic Disulfide C(8) Iminoporfiromycin: Nucleophilic Activation of a Porfiromycin

Significant anti-inflammatory aziridine-containing indole alkaloids from the Chinese medicinal plant Alstonia scholaris

Two unique windmill-like aziridine-containing indole alkaloids, possessing an unprecedented 6/5/5/6/6/5/3 rigid ring system and an unusual azabicyclo[3.1.0]hexane core, were isolated from Alstonia scholaris. Their structures were established by spectroscopy, X-ray diffraction, and electronic circular dichroism calculations. The novel compounds exhibited significant anti-inflammatory bioactivity in vitro and alleviated LPS-induced acute lung injury in mice.

Cyclic 5-membered disulfides are not selective substrates of thioredoxin reductase, but are opened nonspecifically

The cyclic five-membered disulfide 1,2-dithiolane has been widely used in chemical biology and in redox probes. Contradictory reports have described it either as nonspecifically reduced in cells, or else as a highly specific substrate for thioredoxin reductase (TrxR). Here we show that 1,2-dithiolane probes, such as "TRFS" probes, are nonspecifically reduced by thiol reductants and redox-active proteins, and their cellular performance is barely affected by TrxR inhibition or knockout. Therefore, results of cellular imaging or inhibitor screening using 1,2-dithiolanes should not be interpreted as reflecting TrxR activity, and previous studies may need re-evaluation. To understand 1,2-dithiolanes' complex behaviour, probe localisation, environment-dependent fluorescence, reduction-independent ring-opening polymerisation, and thiol-dependent cellular uptake must all be considered; particular caution is needed when co-applying thiophilic inhibitors. We present a general approach controlling against assay misinterpretation with reducible probes, to ensure future TrxR-targeted designs are robustly evaluated for selectivity, and to better orient future research.

Selective, Modular Probes for Thioredoxins Enabled by Rational Tuning of a Unique Disulfide Structure Motif

Specialized cellular networks of oxidoreductases coordinate the dithiol/disulfide-exchange reactions that control metabolism, protein regulation, and redox homeostasis. For probes to be selective for redox enzymes and effector proteins (nM to μM concentrations), they must also be able to resist non-specific triggering by the ca. 50 mM background of non-catalytic cellular monothiols. However, no such selective reduction-sensing systems have yet been established. Here, we used rational structural design to independently vary thermodynamic and kinetic aspects of disulfide stability, creating a series of unusual disulfide reduction trigger units designed for stability to monothiols. We integrated the motifs into modular series of fluorogenic probes that release and activate an arbitrary chemical cargo upon reduction, and compared their performance to that of the literature-known disulfides. The probes were comprehensively screened for biological stability and selectivity against a range of redox effector proteins and enzymes. This design process delivered the first disulfide probes with excellent stability to monothiols yet high selectivity for the key redox-active protein effector, thioredoxin. We anticipate that further applications of these novel disulfide triggers will deliver unique probes targeting cellular thioredoxins. We also anticipate that further tuning following this design paradigm will enable redox probes for other important dithiol-manifold redox proteins, that will be useful in revealing the hitherto hidden dynamics of endogenous cellular redox systems.

Structure-Guided Design, Synthesis, and Biological Evaluation of (2-(1H-Indol-3-yl)-1H-imidazol-4-yl)(3,4,5-trimethoxyphenyl) Methanone (ABI-231) Analogues Targeting the Colchicine Binding Site in Tubulin

ABI-231 is a potent, orally bioavailable tubulin inhibitor that interacts with the colchicine binding site and is currently undergoing clinical trials for prostate cancer. Guided by the crystal structure of ABI-231 in complex with tubulin, we performed structure-activity relationship studies around the 3-indole moiety that led to the discovery of several potent ABI-231 analogues, most notably 10ab and 10bb. The crystal structures of 10ab and 10bb in complex with tubulin confirmed their improved molecular interactions to the colchicine site. In vitro, biological studies showed that new ABI-231 analogues disrupt tubulin polymerization, promote microtubule fragmentation, and inhibit cancer cell migration. In vivo, analogue 10bb not only significantly inhibits primary tumor growth and decreases tumor metastasis in melanoma xenograft models but also shows a significant ability to overcome paclitaxel resistance in a taxane-resistant PC-3/TxR model. In addition, pharmacological screening suggested that 10bb has a low risk of potential off-target function.

Design, synthesis, and mode of action studies of a mitomycin tetramer inducing double activations with a single probe

We report design, synthesis, and mechanistic studies of a new mitomycin tetramer 9 along with a new mitomycin dimer 10. Mitomycin 9 is a tetramer connected by the disulfide linker 11, and easily undergoes disulfide cleavage to provide two dimeric structures 9r that each contains a single thiol probe for activations. So, tetramer 9 as a precursor of 9r was specifically targeted to undergo double activations with a single probe. A tetramer 9 was synthesized using 1 and key intermediate 11, and a dimer 10 was synthesized from 1 and diamine 12. Activation studies revealed that 9 underwent effective double activations with a single probe by nucleophiles while the reference 10 did not. Evaluations of DNA ISC formations showed that 9 generated substantial levels of DNA ISC by nucleophilic activation while the references 10 and 2 did not. The effectiveness of 9 in activation and formation of DNA ISC per probe was verified by comparing with dimers 5-8 of double activations with two probes. These findings highlighted the role of a single thiol in 9r and demonstrated the intended double activations with a single probe, which marks the first case in mitomycin studies.

Studies on synthesis and activation mechanism of mitomycin dimers connected by 1,2-dithiolane and diol linkers

We report the synthetic and mechanistic studies on a new cyclic disulfide mitomycin dimer, 7-N,7'-N'-(1″,2″-dithiolanyl-3″,5″-dimethylenyl)bismitomycin C (8), and a diol mitomycin dimer, 7-N,7'-N'-(2″,4″-dihydroxy-1″,5″-pentanediyl)bismitomycin C (9). Mitomycin 8 is a dimer connected by a 1,2-dithiolane (a five-membered cyclic disulfide) linker, and was specifically designed to undergo nucleophilic activation and double DNA alkylations leading to efficient production of DNA interstrand cross-link (DNA ISC) adducts. Disulfide cleavage in 8 would generate two thiol groups that could serve as probes to activate two mitomycin rings. At first, the target mitomycin 8 was synthesized using mitomycin A (1) and the key intermediate, cyclic disulfide (10), which was prepared through a seven-step synthetic sequence. Diol mitomycin 9 was also synthesized from 1 and diamine salt 13. Next, kinetic studies using solvolysis reaction revealed that the activation rates of 8 were much higher than those of 9 and mitomycin C (2) under nucleophilic conditions provided by Et(3)P presumably due to the presence of a cyclic disulfide unit in 8. These findings led us to propose a nucleophilic activation pathway for 8. Then, DNA ISC experiments further revealed that the levels of DNA ISC caused by 8 in the presence of Et(3)P were much higher (97%) than those by 9 (5%) and 2 (4%). More importantly, mitomycin 8 underwent much faster activation and produced slightly higher levels of DNA ISC than the previously reported mitomycins 5-7. Overall, we concluded that 8 was highly efficient for both nucleophilic activation and corresponding DNA ISC formation, and that this differentiation came from the crucial function of the cyclic disulfide unit in 8.

The macrocycle of leinamycin imparts hydrolytic stability to the thiol-sensing 1,2-dithiolan-3-one 1-oxide unit of the natural product

Reaction of cellular thiols with the 1,2-dithiolan-3-one 1-oxide moiety of leinamycin triggers the generation of DNA-damaging reactive intermediates. Studies with small, synthetic analogues of leinamycin reveal that the macrocyclic portion of the natural product imparts remarkable hydrolytic stability to the 1,2-dithiolan-3-one 1-oxide heterocycle without substantially compromising its thiol-sensing property.

Nucleophilic activation of a tetra-substituted mitomycin cyclic bis-disulfide

The multimerization of functional DNA alkylating agents has drawn significant, recent interest because these compounds are expected to generate enhanced levels of DNA cross-linked adducts, compared with their monomeric agents. Here we report the evaluation of 7-N,7'-N'-(1'',2'',9'',10''-tetrathia-cyclohexadecanyl-3'',8'',11'',16''-tetramethylenyl)tetrakismitomycin C (8), in which four mitomycin units are attached to the novel bis-disulfide linker, 3,8,11,16-tetrakis(aminomethyl)-1,2,9,10-tetrathia-cyclohexadecane. Compound 8 was designed to undergo preferential C(1) mitomycin activation under nucleophilic as well as under acidic and reductive conditions. We anticipated that treating 8 with nucleophiles would lead to bis-disulfide cleavage thus producing two mitomycin dimers (9) capable of generating DNA interstrand cross-links (ISC). The mitomycin units in 9 are tethered by a stable carbon backbone linkage. According to the procedure reported by Lee and coworkers (Tetrahedron, 61, 1749-1754 (2005)), we synthesized 8 and the reference mitomycin dimer, 7-N,7'-N'-(2'',7''-dihydroxy-1'',8''-octanediyl)bismitomycin C (15). Compound 8 was activated under acidic conditions thereby generating mitosene product 16, in which all four mitomycin units within the 16-membered ring were activated. Using the nucleophile Et(3)P, we found that 8 underwent significantly enhanced mitosene production compared with its reference compound 15. We further demonstrated that under nucleophilic activation conditions 8 generated higher levels of DNA ISC than either 1 or 15. The cytotoxicities of 8 and 15 in a select tumor cell line were evaluated and compared with mitomycin C (1).

7-N,7'-N'-(1",2"-Dithianyl-3",6"-dimethylenyl)bismitomycin C: synthesis and nucleophilic activation of a dimeric mitomycin

Dimeric alkylating agents that modify complementary DNA strands have engendered significant interest. We have prepared the novel dimeric mitomycin, 7-N,7'-N'-(1",2"-dithianyl-3",6"-dimethylenyl)bismitomycin C (9), in which the mitomycins are bridged by a dithiane unit. Dimer 9, like the clinically tested acyclic disulfides KW-2149 (3) and BMS-181174 (4), was designed to activate under nucleophilic and reductive conditions. Successive nucleophile-mediated disulfide cleavage transformations of 9 are expected to generate thiol species ideally positioned to render the two mitomycin systems vulnerable to nucleophilic attack and permit DNA interstrand cross-link formation. The dithiane linker, strategically positioned between the two mitomycins, distinguished 9 from 3 and 4. Nucleophilic activation of this cyclic disulfide permitted both activated mitomycins to remain tethered to one another. We report the synthesis of 9, and show that the nucleophile Et(3)P markedly enhances the activation and consumption of 9, compared with the reference compound 7-N, 7"-N'-(cyclohexanyl-trans-1",4"-dimethylenyl)bismitomycin C (27). We further demonstrated that provides higher levels of DNA interstrand cross-links than either the dimeric reference compounds, and 7-N,7-N'-(2",5"-dihydroxy-1",6"-hexanediyl)bismitomycin C (28), or the monomeric mitomycins, 1 and 3, when Et(3)P is added to solutions containing EcoRI-linearized pBR322 DNA.