Stimulation of P-glycoprotein ATPase by analogues of tetramethylrosamine: coupling of drug binding at the "R" site to the ATP hydrolysis transition state

Synthesis of Rhodamines and Rosamines Using 3,6-Difluoroxanthone as a Common Intermediate

Rhodamines and structurally similar rosamines are some of the most highly utilized tools for molecular imaging experiments. We report a general and high-yielding route to produce 18 examples of rhodamines and rosamines, including tetramethylrhodamine, rhodamine B, and Janelia Fluor 549, from a single xanthone intermediate, 3,6-difluoroxanthone. Spectroscopic studies revealed trends in fluorophore efficiency based on substitution patterns at the 3'-, 6'-, and 9'-positions, providing insights to aid future designs of rhodamines/rosamines.

Investigation of chalcogen bioisosteric replacement in a series of heterocyclic inhibitors of tryptophan 2,3-dioxygenase

Selenium is an underexplored element that can be used for bioisosteric replacement of lower molecular weight chalcogens such as oxygen and sulfur. More studies regarding the impact of selenium substitution in different chemical scaffolds are needed to fully grasp this element's potential. Herein, we decided to evaluate the impact of selenium incorporation in a series of tryptophan 2,3-dioxygenase (TDO2) inhibitors, a target of interest in cancer immunotherapy. First, we synthesized the different chalcogen isosteres through Suzuki-Miyaura type coupling. Next, we evaluated the isosteres' affinity and selectivity for TDO2, as well as their lipophilicity, microsomal stability and cellular toxicity on TDO2-expressing cell lines. Overall, chalcogen isosteric replacements did not disturb the on-target activity but allowed for a modulation of the compounds' lipophilicity, toxicity and stability profiles. The present work contributes to our understanding of oxygen/sulfur/selenium isostery towards increasing structural options in medicinal chemistry for the development of novel and distinctive drug candidates.

Selective Conversion of P=O-Bridged Rhodamines into P=O-Rhodols: Solvatochromic Near-Infrared Fluorophores

The substitution of an oxygen atom in rhodols with a phosphine oxide (P=O) moiety affords P=O-bridged rhodols as a new type of near-infrared (NIR) fluorophore. This compound class can be readily accessed upon exposure of the corresponding rhodamines to aqueous basic conditions. The electron-withdrawing effect of the P=O group facilitates the hydrolytic deamination, and, moreover, prolonged exposure to aqueous basic conditions generates P=O-bridged fluoresceins, that is, a series of three P=O-bridged xanthene dyes is available in one simple operation. The P=O-bridged rhodols show significant bathochromic shifts of the longest-wavelength absorption maximum (Δλ=125 nm; >3600 cm^-1 ) upon changing the solvent from toluene to water, whereas the emission is shifted less drastically (Δλ=70 nm; 1600 cm^-1 ). The hydrogen bonding between the P=O and C=O groups with protic solvents results in substantial stabilization of the LUMO level, which is responsible for the solvatochromism.

Selective photodepletion of malignant T cells in extracorporeal photopheresis with selenorhodamine photosensitizers

Extracorporeal photopheresis (ECP) has been used successfully in the treatment of erythrodermic cutaneous T cell lymphoma (CTCL), and other T cell-mediated disorders. Not all patients obtain a significant or durable response from ECP. The design of a selective photosensitizer that spares desirable lymphocytes while targeting malignant T cells may promote cytotoxic T cell responses and improve outcomes after ECP. A series of selenorhodamines built with variations of the Texas red core targeted the mitochondria of malignant T cells, were phototoxic to malignant T cells presumably via their ability to generate singlet oxygen, and were transported by P-glycoprotein (P-gp). To determine the selectivity of the photosensitizers in the ECP milieu, staphylococcal enterotoxin B (SEB)-stimulated and non-stimulated human lymphocytes were combined with HUT-78 cells (a CTCL) to simulate ECP. The amide-containing analogues of the selenorhodamines were transported more rapidly than the thioamide analogues in monolayers of MDCKII-MDR1 cells and, consequently, were extruded more rapidly from P-gp-expressing T cells than the corresponding thioamide analogues. Selenorhodamine 6 with the Texas red core and a piperidylamide functionality was phototoxic to >90% of malignant T cells while sparing >60% of both stimulated and non-stimulated T cells. In the resting T cells, (63±7)% of the CD4+ T cell compartment, and (78±2.5)% of the CD8+ cytotoxic T cell population were preserved, resulting in an enrichment of healthy and cytotoxic T cells after photodepletion.

A comparative study of the photophysics of phenyl, thienyl, and chalcogen substituted rhodamine dyes

We characterize the ultrafast photophysics and electrochemistry of a collection of rhodamine-style dyes and show that different dyes exhibit various directions of charge-transfer in the excited state.

Selenorhodamine photosensitizers for photodynamic therapy of P-glycoprotein-expressing cancer cells

We examined a series of selenorhodamines with amide and thioamide functionality at the 5-position of a 9-(2-thienyl) substituent on the selenorhodamine core for their potential as photosensitizers for photodynamic therapy (PDT) in P-glycoprotein (P-gp) expressing cells. These compounds were examined for their photophysical properties (absorption, fluorescence, and ability to generate singlet oxygen), for their uptake into Colo-26 cells in the absence or presence of verapamil, for their dark and phototoxicity toward Colo-26 cells, for their rates of transport in monolayers of multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells, and for their colocalization with mitochondrial specific agents in Colo-26 cells. Thioamide derivatives 16b and 18b were more effective photosensitizers than amide derivatives 15b and 17b. Selenorhodamine thioamides 16b and 18b were useful in a combination therapy to treat Colo-26 cells in vitro: a synergistic therapeutic effect was observed when Colo-26 cells were exposed to PDT and treatment with the cancer drug doxorubicin.

Pore-exposed tyrosine residues of P-glycoprotein are important hydrogen-bonding partners for drugs

The multispecific efflux transporter, P-glycoprotein, plays an important role in drug disposition. Substrate translocation occurs along the interface of its transmembrane domains. The rotational C2 symmetry of ATP-binding cassette transporters implies the existence of two symmetry-related sets of substrate-interacting amino acids. These sets are identical in homodimeric transporters, and remain evolutionary related in full transporters, such as P-glycoprotein, in which substrates bind preferentially, but nonexclusively, to one of two binding sites. We explored the role of pore-exposed tyrosines for hydrogen-bonding interactions with propafenone type ligands in their preferred binding site 2. Tyrosine 953 is shown to form hydrogen bonds not only with propafenone analogs, but also with the preferred site 1 substrate rhodamine123. Furthermore, an accessory role of tyrosine 950 for binding of selected propafenone analogs is demonstrated. The present study demonstrates the importance of domain interface tyrosine residues for interaction of small molecules with P-glycoprotein.

Chalcogenopyrylium dyes as differential modulators of organic anion transport by multidrug resistance protein 1 (MRP1), MRP2, and MRP4

Multidrug resistance proteins (MRPs) mediate the ATP-dependent efflux of structurally diverse compounds, including anticancer drugs and physiologic organic anions. Five classes of chalcogenopyrylium dyes (CGPs) were examined for their ability to modulate transport of [(3)H]estradiol glucuronide (E(2)17βG; a prototypical MRP substrate) into MRP-enriched inside-out membrane vesicles. Additionally, some CGPs were tested in intact transfected cells using a calcein efflux assay. Sixteen of 34 CGPs inhibited MRP1-mediated E(2)17βG uptake by >50% (IC50 values: 0.7-7.6 µM). Of 9 CGPs with IC50 values ≤2 µM, two belonged to class I, two to class III, and five to class V. When tested in the intact cells, only 4 of 16 CGPs (at 10 µM) inhibited MRP1-mediated calcein efflux by >50% (III-1, V-3, V-4, V-6), whereas a fifth (I-5) inhibited efflux by just 23%. These five CGPs also inhibited [(3)H]E(2)17βG uptake by MRP4. In contrast, their effects on MRP2 varied, with two (V-4, V-6) inhibiting E(2)17βG transport (IC(50) values: 2.0 and 9.2 µM) and two (V-3, III-1) stimulating transport (>2-fold), whereas CGP I-5 had no effect. Strikingly, although V-3 and V-4 had opposite effects on MRP2 activity, they are structurally identical except for their chalcogen atom (Se versus Te). This study is the first to identify class V CGPs, with their distinctive methine or trimethine linkage between two disubstituted pyrylium moieties, as a particularly potent class of MRP modulators, and to show that, within this core structure, differences in the electronegativity associated with a chalcogen atom can be the sole determinant of whether a compound will stimulate or inhibit MRP2.

Thiorhodamines containing amide and thioamide functionality as inhibitors of the ATP-binding cassette drug transporter P-glycoprotein (ABCB1)

Twelve thiorhodamine derivatives have been examined for their ability to stimulate the ATPase activity of purified human P-glycoprotein (P-gp)-His(10), to promote uptake of calcein AM and vinblastine into multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells, and for their rates of transport in monolayers of multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells. The thiorhodamine derivatives have structural diversity from amide and thioamide functionality (N,N-diethyl and N-piperidyl) at the 5-position of a 2-thienyl substituent on the thiorhodamine core and from diversity at the 3-amino substituent with N,N-dimethylamino, fused azadecalin (julolidyl), and fused N-methylcyclohexylamine (half-julolidyl) substituents. The julolidyl and half-julolidyl derivatives were more effective inhibitors of P-gp than the dimethylamino analogues. Amide-containing derivatives were transported much more rapidly than thioamide-containing derivatives.

Rhodamine inhibitors of P-glycoprotein: an amide/thioamide "switch" for ATPase activity

We have examined 46 tetramethylrosamine/rhodamine derivatives with structural diversity in the heteroatom of the xanthylium core, the amino substituents of the 3- and 6-positions, and the alkyl, aryl, or heteroaryl group at the 9-substituent. These compounds were examined for affinity and ATPase stimulation in isolated MDR3 CL P-gp and human P-gp-His(10), for their ability to promote uptake of calcein AM and vinblastine in multidrug-resistant MDCKII-MDR1 cells, and for transport in monolayers of MDCKII-MDR1 cells. Thioamide 31-S gave K(M) of 0.087 microM in human P-gp. Small changes in structure among this set of compounds affected affinity as well as transport rate (or flux) even though all derivatives examined were substrates for P-gp. With isolated protein, tertiary amide groups dictate high affinity and high stimulation while tertiary thioamide groups give high affinity and inhibition of ATPase activity. In MDCKII-MDR1 cells, the tertiary thioamide-containing derivatives promote uptake of calcein AM and have very slow passive, absorptive, and secretory rates of transport relative to transport rates for tertiary amide-containing derivatives. Thioamide 31-S promoted uptake of calcein AM and inhibited efflux of vinblastine with IC(50)'s of approximately 2 microM in MDCKII-MDR1 cells.

Chalcogenopyrylium dyes as inhibitors/modulators of P-glycoprotein in multidrug-resistant cells

A series of chalcogenopyrylium dyes were evaluated as modulators/inhibitors of P-glycoprotein (Pgp). Their ability to inhibit verapamil (VER)-dependent ATPase activity (IC(50) values) in lipid-activated, mouse Cys-less mdr3 Pgp was determined. Their ability to promote calcein-AM (CAM) uptake in MDCKII-MDR1 cells and their capacity to be transported by Pgp in monolayers of MDCKII-MDR1 cells were also evaluated. The chalcogenopyrylium dyes promoted CAM uptake with values of EC(50) between 5 x 10(-6) and 3.5 x 10(-5)M and 7 of the 9 dyes examined in transport studies were substrates for Pgp with efflux ratios (P(BA/AB)) between 14 and 390. Binding of three compounds (1-S, 3-S, and 4-S) to Pgp was also assessed by fluorescence. These three thiopyrylium dyes showed increased fluorescence upon binding to Pgp, giving apparent binding constants, K(app), on the order of 10(-7) to 10(-6)M. Compound 8-Te was particularly intriguing since it appeared to influence Pgp at low micromolar concentrations as evidenced by its influence on VER-stimulated ATPase activity (IC(50) of 1.2 x 10(-6)M), CAM uptake (EC(50) of 5.4 x 10(-6)M), as well as [(3)H]-vinblastine transport by Pgp in cells (IC(50) of 4.3 x 10(-6)M) and within inside-out membrane vesicles (IC(50) of 9.6 x 10(-6)M). Yet, Pgp did not influence the distribution of 8-Te in MDCKII-MDR1 monolayers suggesting that 8-Te may bind to an allosteric site.

ATP occlusion by P-glycoprotein as a surrogate measure for drug coupling

The multidrug efflux pump P-glycoprotein (Pgp) couples drug transport to ATP hydrolysis. Previously, using a synthetic library of tetramethylrosamine ( TMR) analogues, we observed significant variation in ATPase stimulation ( V m (D)). Concentrations required for half-maximal ATPase stimulation ( K m (D)) correlated with ATP hydrolysis transition-state stabilization and ATP occlusion (EC 50 (D)) at a single site. Herein, we characterize several TMR analogues that elicit modest turnover ( k cat <or= 1-2 s (-1)) compared to verapamil (VER) ( k cat approximately 10 s (-1)). Apparent ATPase activities manifest as nearly equivalent to basal values. In some cases, K m (D) parameters for drug stimulation of ATPase could not be accurately determined, yet these same TMR analogues promoted ATP occlusion at relatively low concentrations ( approximately 0.4-40 microM). Moreover, the TMR analogues competitively inhibited VER-dependent ATPase activity at concentrations similar to those required for ATP occlusion. Finally, the TMR analogues facilitated uptake of calcein-AM into CR1R12 and MDCK-MDR1 cells and are actively transported by Pgp in monolayers of MDCK-MDR1 cells at similarly low concentrations ( approximately 1-20 microM). ADP.V i release kinetics were identical in the presence of the TMR derivatives, VER, or in the absence of drug, suggesting that slow turnover is not likely due to slow release of the ATP hydrolysis products ADP and P i. These data support the partition model in which drug site occupancy converts residual basal ATPase activity to a drug-dependent mechanism even in cases where stimulation appears to be exactly compensatory to basal values. It is noteworthy that when compared to previously reported TMR analogues, subtle modification of the TMR scaffold can confer large differences in ATP turnover.

About a switch: how P-glycoprotein (ABCB1) harnesses the energy of ATP binding and hydrolysis to do mechanical work

The efflux of drugs by the multidrug transporter P-glycoprotein (Pgp; ABCB1) is one of the principal means by which cancer cells evade chemotherapy and exhibit multidrug resistance. Mechanistic studies of Pgp-mediated transport, however, transcend the importance of this protein per se as they help us understand the transport pathway of the ATP-binding cassette proteins in general. The ATP-binding cassette proteins comprise one of the largest protein families, are central to cellular physiology, and constitute important drug targets. The functional unit of Pgp consists of two nucleotide-binding domains (NBD) and two transmembrane domains that are involved in the transport of drug substrates. Early studies postulated that conformational changes as a result of ATP hydrolysis were transmitted to the transmembrane domains bringing about drug transport. More recent structural and biochemical studies on the other hand suggested that ATP binds at the interface of the two NBDs and induces the formation of a closed dimer, and it has been hypothesized that this dimerization and subsequent ATP hydrolysis powers transport. Based on the mutational and biochemical work on Pgp and structural studies with isolated NBDs, we review proposed schemes for the catalytic cycle of ATP hydrolysis and the transport pathway.

A microwave-assisted synthesis of julolidine-9-carboxamide derivatives and their conversion to chalcogenoxanthones via directed metalation

9-formyljulolidine was oxidized via a microwave-assisted Willgerodt-Kindler reaction to the N-piperidine or N-morpholine julolidine-9-thioamide. 9-formyl-1,1,7,7-tetramethyljulolidine gave the corresponding N-piperidine tetramethyljulolidine-9-thioamide. The thioamides were converted to the corresponding carboxamides with trifluoroacetic anhydride. The amide group directed ortho-metalation in the julolidine system, but not in the tetramethyljulolidine system. The resulting anion was captured by dichalcogenide electrophiles. The resulting products were converted to chalcogenoxanthones with phosphorus oxychloride and triethylamine (POCl3/Et3N).

Generation of 3- and 5-Lithiothiophene-2-carboxylates via Metal−Halogen Exchange and Their Addition Reactions to Chalcogenoxanthones

Deprotonation and lithium-bromine exchange in 5- or 3-bromothiophene-2-carboxylic acids with t-BuLi form the corresponding dianion, which reacts highly regioselectively in the presence of 0.25 equiv of tetramethyl-1,2-ethylenediamine with 3,6-bis(dimethylamino) chalcogenoxanthones to give S- and Se-containing rhodamines. Quenching studies with D2O indicate that an extra equivalent of t-BuLi is not necessary in these reactions. Deprotonation is faster than metal-halogen exchange with the bromothiophene-2-carboxylic acids using t-BuLi.

A cationic chalcogenoxanthylium photosensitizer effective in vitro in chemosensitive and multidrug-resistant cells

Pentacyclic thio- (1) and seleno- (2) analogues of tetramethylrosamine (TMR) were prepared with a julolidyl fragment replacing the 3-dimethylamino substituent in the xanthylium core. The pentacylic structure increases the lipophilicity of 1 and 2 relative to TMR-S and TMR-Se and locks the lone-pair of electrons on the julolidyl N atom into conjugation with the xanthylium core. This conformational rigidization leads to longer wavelengths of absorption, but has little impact on other photophysical properties such as quantum yields for fluorescence and singlet-oxygen generation and fluorescence lifetimes in 1 and 2 relative to TMR-S and TMR-Se. Both 1 and 2 are effective photosensitizers against chemosensitive AUXB1 cells in vitro at 1x10(-7)M and compound 2 is an effective photosensitizer against multidrug-resistant CR1R12 cells in vitro at 1x10(-7)M. While the uptake TMR-S into CR1R12 cells as measured by fluorescence is significantly lower than uptake into chemosensitive AUXB1 cells, there is no significant difference in the uptake of 1 into either AUXB1 or CR1R12 cells. The addition of 2x10(-4)M verapamil to the cells prior to treatment with 1 had no significant effect on the uptake of 1 into either AUXB1 or CR1R12 cells. Treating lipid-activated, purified Pgp with 2 and light gave complete inhibition of Pgp ATPase activity.