A family of lipophilic, cationic Au(I) complexes of N-heterocyclic carbenes (NHCs) have been designed as new mitochondria-targeted antitumor agents that combine both selective mitochondrial accumulation and selective thioredoxin reductase inhibition properties within a single molecule. Two-step ligand exchange reactions with cysteine (Cys) and selenocysteine (Sec) occur with release of the NHC ligands. At physiological pH the rate constants for the reactions with Sec are 20- to 80-fold higher than those with Cys. The complexes are selectively toxic to two highly tumorigenic breast cancer cell lines and not to normal breast cells, and the degree of selectivity and potency are optimized by modification of the substituent on the simple imidazolium salt precursor. The lead compound is shown to accumulate in mitochondria of cancer cells, to cause cell death through a mitochondrial apoptotic pathway and to inhibit the activity of thioredoxin reductase (TrxR) but not the closely related and Se-free enzyme glutathione reductase.

Reported herein is a study of the unusual 3′-3′ 1,4-GG interstrand cross-link (IXL) formation in duplex DNA by a series of polynuclear platinum anticancer complexes. To examine the effect of possible preassociation through charge and hydrogen-bonding effects the closely related compounds [{trans-PtCl(NH 3 ) 2 } 2 (μ-trans-Pt(NH 3 ) 2 -{NH 2 (CH 2 ) 6 NH 2 } 2 )] 4+ (BBR3464, 1), [{trans-PtCl (NH 3 ) 2 } 2 (μ-NH 2 (CH 2 ) 6 -NH 2 )] 2+ (BBR3005, 2), [{trans-PtCl-(NH 3 ) 2 } 2 (μ-H 2 N (CH 2 ) 3 NH 2 (CH 2 ) 4 )] 3+ (BBR3571, 3) and [{trans-PtCl(NH 3 ) 2 } 2 -{μ-H 2 N(CH 2 ) 3 -N(COCF 3 ) (CH 2 ) 4 }] 2+ (BBR3571-COCF 3 , 4) were studied. Two different molecular biology approaches were used to investigate the effect of DNA template upon IXL formation in synthetic 20-base-pair duplexes. In the "hybridisation directed" method the monofunctionally adducted top strands were hybridised with their complementary 5′-end labelled strands; after 24 h the efficiency of interstrand cross-linking in the 5′-5′ direction was slightly higher than in the 3′-3′ direction. The second method involved "postsynthetic modification" of the intact duplex; significantly less cross-linking was observed, but again a slight preference for the 5′-5′ duplex was present. 2D [ 1 H, 15 N] HSQC NMR spectroscopy studies of the reaction of [ 15 N]-1 with the sequence 5′-d{TATAC-ATGTATA} 2 allowed direct comparison of the stepwise formation of the 3′-3′ IXL with the previously studied 5′-5′ IXL on the analogous sequence 5′-d(ATATGTACATAT) 2 . Whereas the preassociation and aquation steps were similar, differences were evident at the monofunctional binding step. The reaction did not yield a single distinct 3′-3′ 1,4-GG IXL, but numerous cross-linked adducts formed. Similar results were found for the reaction with the dinuclear [ 15 N]-2. Molecular dynamics simulations for the 3′-3′ IXLs formed by both 1 and 2 showed a highly distorted structure with evident fraying of the end base pairs and considerable widening of the minor groove.

The aquation profiles of two novel dinuclear polyamine-linked, platinum-based antitumour complexes [{trans-PtCl(15NH3)2}2{μ-(15NH2(CH2)615NH2(CH2)615NH2)}]3+ (BBR3007, 1,1/t,t-6,6, 1) and [{trans-PtCl(15NH3)2}2{μ-(15NH2(CH2)615NH2(CH2)215NH2(CH2)615NH2)}]4+ (BBR3610, 1,1/t,t-6,2,6, 1′) have been probed using 2D [1H, 15N] HSQC NMR spectroscopy. Reported herein are the rate constants for the hydrolysis of 1 and 1′, as well as the acid dissociation constants of the coordinated aqua ligands in their aquated derivatives. The aquation and anation rate constants for the single step aquation model in 15 mM NaClO4 (pH 5.4) at 298 K are, for 1, k1 = 7.2 ± 0.1 ×10−5 s−1, k−1 = 0.096 ± 0.002 M−1 s−1 and, for 1′, k1 = 4.0 ± 0.2 × 10−5 s−1, k−1 = 1.4 ± 0.1 M−1 s−1. The effect of the linker backbone (Pt(tetra(m)mine vs. polyamine) was evaluated by comparison with previous data for the trinuclear complex [{trans-PtCl(NH3)2}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)]4+ (1,0,1/t,t,tor BBR3464). The pK1 for 1,0,1/t,t,t(3.44) is closest to that of 1 (3.12), while the pronounced difference for 1′ (4.54), means that 1′ is the least aquated of the three complexes at equilibrium. pKa values of 5.92 were calculated for the aquated forms of both 1 and 1′, which are 0.3 pKunits higher than for either 1,0,1/t,t,t, or the dinuclear 1,1/t,t. The higher pKa values for both polyamine-linked compounds may be attributed to the formation of macrochelates between the central NH2 groups and the {PtN3O}coordination sphere of the aquated species.

Two-dimensional (2D)[1H, 15N] heteronuclear single-quantum coherence (HSQC) NMR experiments of the kinetics of aquation and sulfation of the dinuclear platinum anticancer complex [{trans-PtCl(NH3)2}2(μ-NH2(CH2)6NH2)]2+ (1,1/t,t, 1) in 15 mM sulfate solution are reported using conditions (298 K, pH 5.4) identical to those previously used for other anionic systems (phosphate and acetate), allowing for a direct comparison. Sulfate is the fourth most abundant anion in human plasma. The rate constant for the aquation step (kH) is higher than that previously found in the presence of phosphate, but the anation rate constants are similar. The rate constant for sulfate displacement of the aqua ligand (kL) is approximately three times higher than that of phosphate, and a further major difference between these two anions is the very high k−L for loss of sulfate, suggesting that when formed in plasma the sulfato species will be substitution labile. We also introduce a novel (free) plug-in, ‘2D NMR analysis’, developed for the expedited integration and analysis of 2D [1H, 15N] HSQC NMR spectra. We have found that this plug-in significantly reduces the amount of time taken in the analysis of experiments with no loss to the quality of the data.