Identification of cyclosporin C from Amphichorda felina using a Cryptococcus neoformans differential temperature sensitivity assay

We used a temperature differential assay with the opportunistic fungal pathogen Cryptococcus neoformans as a simple screening platform to detect small molecules with antifungal activity in natural product extracts. By screening of a collection extracts from two different strains of the coprophilous fungus, Amphichorda felina, we detected strong, temperature-dependent antifungal activity using a two-plate agar zone of inhibition assay at 25 and 37 °C. Bioassay-guided fractionation of the crude extract followed by liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance spectroscopy (NMR) identified cyclosporin C (CsC) as the main component of the crude extract responsible for growth inhibition of C. neoformans at 37 °C. The presence of CsC was confirmed by comparison with a commercial standard. We sequenced the genome of A. felina to identify and annotate the CsC biosynthetic gene cluster. The only previously characterized gene cluster for the biosynthesis of similar compounds is that of the related immunosuppressant drug cyclosporine A (CsA). The CsA and CsC gene clusters share a high degree of synteny and sequence similarity. Amino acid changes in the adenylation domain of the CsC nonribosomal peptide synthase's sixth module may be responsible for the substitution of L-threonine compared to L-α-aminobutyric acid in the CsA peptide core. This screening strategy promises to yield additional antifungal natural products with a focused spectrum of antimicrobial activity.

Potent nonimmunosuppressive cyclophilin inhibitors with improved pharmaceutical properties and decreased transporter inhibition

Nonimmunosuppressive cyclophilin inhibitors have demonstrated efficacy for the treatment of hepatitis C infection (HCV). However, alisporivir, cyclosporin A, and most other cyclosporins are potent inhibitors of OATP1B1, MRP2, MDR1, and other important drug transporters. Reduction of the side chain hydrophobicity of the P4 residue preserves cyclophilin binding and antiviral potency while decreasing transporter inhibition. Representative inhibitor 33 (NIM258) is a less potent transporter inhibitor relative to previously described cyclosporins, retains anti-HCV activity in cell culture, and has an acceptable pharmacokinetic profile in rats and dogs. An X-ray structure of 33 bound to rat cyclophilin D is reported.

Differentiation of cyclosporin A from isocyclosporin A by liquid chromatography/electrospray ionization mass spectrometry with post-column addition of divalent metal salt

RATIONALE

Cyclosporin A (CsA) rearranges to its isomer isocyclosporin A (isoCsA) upon acid hydrolysis and also during ionization in the ion source of the mass spectrometer. It has been reported that both compounds could not be differentiated by tandem mass spectrometry (MS/MS) using atmospheric pressure ionization (API) sources and ambiguously differentiated by using other sources. In order to analyze these compounds which are common fungal metabolites, it is relevant to develop a simple method for their differentiation.

METHODS

CsA and isoCsA were analyzed by liquid chromatography/mass spectrometry (LC/MS) with post-column addition of metal ion solutions in a quadrupole time-of-flight instrument equipped with an electrospray ionization (ESI) source.

RESULTS

Mass spectra of CsA obtained upon post-column addition of solutions of Ca(II), Cu(II) and Zn(II) showed complexes between cyclosporin and the metal, including [2CsA + Me](2+) and [CsA-H + Me](+). These complexes were not observed in the spectra of isoCsA. The same results were observed at different metal concentrations.

CONCLUSIONS

Differentiation via metal complexation in positive ion mode LC/ESI-MS was performed to simultaneously distinguish CsA and its isomer isoCsA.

Cyclosporins from Mycelium sterilae MS 2929

The structures of two new cyclosporins were elucidated by NMR and MS methods as cyclo[-MeBmt(1)-Abu(2)-Sar(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-d-Ala(8)-MeLeu(9)-MeNva(10)-MeVal(11)-] and cyclo[-MeBmt(1)-Abu(2)-Sar(3)-MeLeu(4)-Abu(5)-MeLeu(6)-Ala(7)-d-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)-].

Biological conversion of a water-soluble prodrug of cyclosporine A

UNIL088 is a water-soluble prodrug of cyclosporine A (CsA) designed for topical ocular delivery. The pro-moiety is grafted via an ester function to CsA and the solubilizing group is a phosphate ion. The aim of this study was to elucidate the conversion mechanisms by which UNIL088 generates CsA. UNIL088 was incubated in rabbit tears at physiological temperature to study its enzymatic and chemical conversion, respectively. Metabolites and intermediates were identified using a quadrupole-time of flight (QqTOF) mass spectrometer, which allowed biotransformation pathways to be deduced. Conversion is activated by the chemical or enzymatic hydrolysis of the terminal ester function of the pro-moiety, leading to the phospho-serine-sarcosine-cyclosporine A that spontaneously converts into CsA. In addition to the main biotransformation pathway, a secondary reaction involved hydrolysis of the phosphate ester group of the pro-moiety, probably by phosphatases present in tears.

A fluorescence polarization-based assay for peptidyl prolyl cis/trans isomerase cyclophilin A

Peptidyl prolyl cis/trans isomerase cyclophilin A (CypA) serves as a cellular receptor for the important immunosuppressant drug, cyclosporin A. In addition, CypA and its enzyme family have been found to play critical roles in a variety of biological processes, including protein trafficking, HIV and HCV infection/replication, and Ca(2+)-mediated intracellular signaling. For these reasons, cyclophilins have emerged as potential drug targets for several diseases. Therefore, it is extremely important to screen for novel small molecule cyclophilin inhibitors. Unfortunately, the biochemical assays reported so far are not adaptable to a high-throughput screening format. Here, we report a fluorescence polarization-based assay for human CypA that can be adapted to high-throughput screening for drug discovery. The technique is based on competition and uses a fluorescein-labeled cyclosporin A analog and purified human CypA to quantitatively measure the binding capacity of unlabeled inhibitors. Detection by fluorescence polarization allows real-time measurement of binding ratios without separation steps. The results obtained demonstrated significant correlation among assay procedures, suggesting that the application of fluorescence polarization in combination with CypA is highly advantageous for the accurate assessment of inhibitor binding.

Exo-mechanism proximity-accelerated alkylations: investigations of linkers, electrophiles and surface mutations in engineered cyclophilin-cyclosporin systems

Investigations on the scope and utility of exo-mechanism proximity-accelerated reactions in engineered receptor-ligand systems are reported. We synthesized a series of electrophilic cyclosporin (CsA) derivatives by varying electrophiles and linker lengths, prepared a series of nucleophilic cysteine mutations on the surface of cyclophilin A (Cyp), and examined their reactivity and specificity in proximity-accelerated reactions. Acrylamide and epoxide electrophiles afforded useful reactivity and high specificity for alkylation of engineered receptors in Jurkat cell extracts. We found that remote cysteines (>17 A from the ligand) could be alkylated with useful rates under physiological conditions. The results from mutations of the receptor surface suggest that the dominant factors governing the rates of proximity-accelerated reactions are related to the local environment of the reactive group on the protein surface. This study defines several parameters affecting reactivity in exo-mechanism proximity-accelerated reactions and provides guidance for the design of experiments for biological investigations involving proximity-accelerated reactions.

Novel cyclosporin derivatives featuring enhanced skin penetration despite increased molecular weight

Topical cyclosporin A (CsA, 1) is not effective in the treatment of skin diseases, due to its low skin penetration. Following a prodrug strategy, a series of novel derivatives of 1 and of 2-[O-(2-hydroxyethyl)-d-Ser(8)]-CsA (SDZ IMM 125, 5) with potentially enhanced skin penetration properties were synthesized, in order to achieve higher levels of the active parent drugs in the skin. Permeation through skin and prodrug/drug levels in the skin were measured in vitro using rat and human skin. Introduction of a polar side chain, either in the form of a positively charged quaternary amine, a negatively charged phosphate or sulfate, or an amphiphilic phosphocholine moiety, generally increased permeability. Maximal increase in permeability through skin relative to CsA was up to 300-fold with rat skin, and up to 16-fold with human skin. Penetration into skin, as evaluated by measurement of prodrug/drug concentrations in the skin after 48 h, could be enhanced up to 14-fold (rat and human skin). Increases of permeation rates and skin concentrations showed no strict correlation. Using the phosphate 10 as prodrug, a 2.5-fold higher concentration of the active parent compound (5) could be achieved in rat skin as when administering 5 itself. The results demonstrate that in contrast with the '500 Dalton rule', which postulates poor skin penetration of molecules larger than 500 Da, high skin permeation can be achieved also with compounds of a molecular weight in the range between 1200 and 1600 Da. Results also indicate that in principle higher skin levels of active drug can be attained with a prodrug strategy in this class of compounds.

In vitro anti-parasitic activity of Cyclosporin A analogs on Trypanosoma cruzi

Cyclosporin A (CsA) nonimmunosuppressive analogs were evaluated against Trypanosoma cruzi and on TcCyP19, a cyclophilin of 19 kDa. Two out of eight CsA analogs, H-7-94 and F-7-62 showed the best anti-parasitic effects on all in vitro assays. Their IC(50) values were 0.82 and 3.41 microM, respectively, compared to CsA IC(50) value 5.39 microM on epimastigote proliferation; and on trypomastigote lysis their IC(50) values were 0.97 and 2.66 microM compared to CsA IC(50) value 7.19 microM. H-7-94 and F-7-62 were also more effective than CsA in inhibiting trypomastigote infection. The enzymatic activity of TcCyP19 was inhibited by all CsA derivatives, suggesting this target is involved in the trypanocidal effects observed.

Design and synthesis of chromogenic thiopeptolide substrates as MetAPs active site probes

Twenty one chromogenic thiopeptolide substrates were designed and synthesized as the active site probes and analyzed with each S1 site of mutant residues and enzymes of wild-type MetAP1s. The preliminary enzymatic experiments indicate that cysteine 70 or 202, at either Escherichia coli or human MetAP1, played a crucial role in the methionine hydrolysis.

Solution conformations of cyclosporins and magnesium-cyclosporin complexes determined by vibrational circular dichroism

Vibrational circular dichroism (VCD) spectroscopy was used to investigate the solution conformations of cyclosporins A, C, D, G, and H in CDCl(3), in the amide I and NH/OH-stretching regions, and their corresponding magnesium complexes in CD(3)CN, in the amide I region. VCD spectra are sensitive to the chiral arrangement of Cdbond;O and NH bonds in this cyclic undecapeptide. Calculations of molecular geometries, as well as IR and VCD intensities of model cyclosporin fragments that include the intramolecular hydrogen bonds of the crystal conformations of cyclosporins A and H (CsA and CsH), were carried out at the density functional theory (DFT; BPW91 functional/6-31G* basis set) level. The good agreement between IR and VCD spectra from experiment and DFT calculations provides evidence that the crystal conformation of CsA is dominant in CDCl(3) solution; CsH, however, assumes both an intramolecularly hydrogen-bonded crystal conformation and more open forms in solution. Comparisons of the experimental and calculated VCD spectra in the NH/OH-stretching region of the noncomplexed cyclosporins indicate that conformers with both free and hydrogen-bonded NH and OH groups are present in solution. Differences between the IR and VCD spectra for the metal-free and magnesium-complexed cyclosporins are indicative of strong interactions between cyclosporins and magnesium ions.

Synthesis and biological evaluation of novel cyclosporin a analogues: potential soft drugs for the treatment of autoimmune diseases

Cyclosporin A is effective in the treatment of asthma patients, but its chronic use is limited by toxicity. Novel cyclosporin A analogues were synthesized utilizing the olefin metathesis reaction and evaluated in a calcineurin A inhibition assay. The novel analogues demonstrated activity comparable to activity of the parent molecule and are potential soft drugs.

Cyclosporins: structure-activity relationships for the inhibition of the human FPR1 formylpeptide receptor

The human formylpeptide receptor (FPR) is a seven-transmembranous G-protein-coupled receptor (7TM-GPCR) for chemotactic peptides of bacterial origins, possibly involved in the recruitment and activation of neutrophils in various inflammatory diseases of mucosal epithelia. Mutational analyses suggest that interactions of formylated peptides with FPR occur on the outer exoplasmic leaflet/domains of the plasma membrane. The immunosuppressive and antifungal antibiotic cyclic undecapeptide cyclosporin A (CsA; cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)]) and some tested analogues such as [Ala(2)]-CsA, [Thr(2)]-CsA, [Val(2)]-CsA, and [Nva(2)]-CsA were able of inhibiting the binding of formylpeptides to the FPR, with [D-MeVal(11)]-CsA (CsH) being much more active than the other analogues. CsH is devoid of immunosuppressive and antifungal activities, and its large potency for human FPR inhibition is of inverse agonism origin. Formylpeptide binding to FPR-expressing cells does not only induce chemotaxis; it also causes a rapid release of granule enzymes in the extracellular medium, allowing the easy monitoring of any inhibition of FPR function "in vivo" (with intact live cells). With such an assay, CsH was confirmed to be the most potent FPR inhibitory cyclosporin, although a far related immunosuppressive cyclosporin analogue, FR901459 ([Thr(2), Leu(5), Leu(10)]-CsA), was found to display a high FPR inhibitory activity (FPR-InhA). To establish structure-activity relationships (SAR) for FPR function inhibition, 59 cyclosporins were now studied by this standardized assay (with differentiated human leukemic cell line HL-60 as FPR-expressing cells and with N-acetyl-beta-D-glucosaminidase release as read-out). These SAR confirmed the low FPR-InhA of classical cyclosporins, where such activity was only seldom found: the most active ones ([Thr(2), Ile(5)]-CsA, [aMeIle(11)]-CsA, and [MeAla(11)]-CsA) remained 3-10-fold less potent than CsH. In contrast, the SAR disclosed that N(10)-desmethylated cyclosporins were particularly prone to display a large FPR-InhA: their most potent one was a [Thr(2), Gly(3), Leu(5), D-Hiv(8), Leu(10)]-CsA, found to be only 2-4-fold less active than [D-MeVal(11)]-CsA (CsH), with which it shows six differences out of 11 residues. Because the free conformations of both CsH and N(10)-desmethylated cyclosporins differ from those of "classical" (N(10)-methylated, [L-MeVal(11)]-using) cyclosporins, these potent FPR inhibitory cyclosporins probably bind to FPR pharmacophores for which classical cyclosporins show little affinity. Moreover, because the conformations of the N(10)-desmethylated cyclosporins widely differ from the CsH one, they probably bind to different pharmacophores on the FPR molecules.

Cyclosporins: structure-activity relationships for the inhibition of the human MDR1 P-glycoprotein ABC transporter

Cyclic undecapeptide cyclo-[MeBmt(1)-Abu(2)-MeGly(3)-MeLeu(4)-Val(5)-MeLeu(6)-Ala(7)-D-Ala(8)-MeLeu(9)-MeLeu(10)-MeVal(11)], the immunosuppressive and antifungal antibiotic cyclosporin A (CsA), was reported to interfere with the MDR1 P-glycoprotein (Pgp), a transmembranous adenosine 5'-triphosphate binding cassette (ABC) transporter with phospholipid flippase or "hydrophobic vacuum cleaner" properties that mediate multidrug resistance (MDR) of cancer cells. By use of photoaffinity-labeled cyclosporins and membranes from Pgp-expressing cells, it was recently shown that in vitro, Pgp molecules could bind a large cyclosporin domain involving residues 4-9 as well as the side chain of residue 1. Tumor cell MDR can also be reversed by a product more distantly related to cyclosporin with the structure [Thr(2), Leu(5), D-Hiv(8), Leu(10)]-CsA (SDZ 214-103). In a standardized assay that measures Pgp function in vivo (on intact live cells) by the Pgp-mediated efflux of the calcein-AM Pgp substrate and uses human lymphoblastoid MDR-CEM (VBL(100)) cells as highly resistant Pgp-expressing cells, SDZ 214-103 was found to be one of the most active Pgp inhibitors among naturally occurring cyclosporins, with an IC(50) of 1.6 microM in an assay where CsA gives an IC(50) of 3.4 microM. Using the in vivo assay, 60, mostly natural, cyclosporin analogues were analyzed to establish structure-activity relationships (SAR). Our SAR are compatible with the in vitro-defined Pgp binding domain model and further disclose that in vivo Pgp inhibition is favored by larger hydrophobic side chains on cyclosporin residues 1, 4, 6, and 8 and a smaller one on residue 7, although with no effect on the residue 5 side chain; moreover, larger hydrophobic side chains on other residues 2, 3, 10, and 11 (outside the in vitro-defined Pgp binding domain) also favor the eventual inhibition of Pgp function. The N-desmethylation of any of the seven N-methylated amides, as naturally occurring in numerous cyclosporins, regularly leads to a decreased Pgp inhibitory activity (Pgp-InhA), up to its abrogation if it occurs at residues 4 and 9. Nevertheless, despite unfavorable use of [Thr(2)] and [Leu(10)] residues, all [D-Hiv(8)] analogues whose lead is SDZ 214-103 show a large Pgp-InhA. The SAR for Pgp inhibition by cyclosporins are thus very complex. Because CsA and SDZ 214-103 show largely different conformations when free in solution, but remarkably similar ones when bound to the cytosolic cyclophilins, SAR for Pgp inhibition must similarly include requirements for occurrence of suitable conformers for insertion in the cell membrane, sufficient conformational plasticity for gaining access to Pgp binding sites, and an adequate conformer structure there to achieve such binding with a high enough affinity and possibly escape from sequestration on cyclophilins.

Interactions of cyclosporines with lipid membranes as studied by solid-state nuclear magnetic resonance spectroscopy and high-sensitivity titration calorimetry

Cyclosporin A (CyA) interacts with lipid membranes. Binding reaction and membrane location of CyA and analogs were examined with 2H-NMR, high-sensitivity isothermal titration calorimetry (ITC), and CD spectroscopy. Effects of CyA and charged analogs on the phosphocholine head group and on the membrane interior were investigated using selectively deuterated phospholipids. Incorporation of cyclosporin generated small disordering of the lipid acyl chains. Binding of CyA and neutral and positively charged analogs to lipid membranes showed endothermic heats of reaction between + 5.9 and + 11.3 kcal/mol, whereas enthalpy of binding was close to zero for the negatively charged derivative. Binding constants of cyclosporines to liposomal membranes were in the range of K(P) = 1650-5560 M(- 1) depending on the cholesterol content. (2)H-NMR provides evidence that CyA is essentially located in the interior of the bilayer membrane. For the charged analogs an additional interaction occurs at the head group level, placing the polar groups of these CyA analogs in the vicinity of the phosphocholine dipoles. The association of CyA and its analogs is accompanied by a positive enthalpy change, which is overcompensated by positive entropy changes. Binding of CyA to lipid membranes thus follows the classical hydrophobic effect, which is in contrast to many other peptide-lipid binding reactions.

Role of amino acid N-methylation in cyclosporins on ring opening and fragmentation mechanisms during collisionally induced dissociation in an ion trap

The product ion mass spectra (collisionally induced dissociation mass spectra) of 12 different cyclosporins modified at every N-methylated amino acid residue with respect to cyclosporin A were compared and the effect of N-demethylation on ring opening mechanisms was evaluated. The four preferential protonation sites were identified in [MeBmt(1)]-cyclosporins. Three sites represented the N-methylated nitrogens of Sar(3), MeLeu(6) and MeLeu(9), while the remaining one represented the lactone group formed by the intramolecular N,O-acyl shift. Selective N-demethylation resulted either in the deletion of the entire fragment ion series or its substantial attenuation. The structures of three new natural cyclosporins in the study were supported by NMR data.

11-Demethylcyclosporins exhibit a single conformation in methanol and dimethylsulfoxide

NMR studies showed that 11-demethylcyclosporin A (cyclosporin E) and 11-demethylcyclosporin B exist as single species both in polar and nonpolar solvents. They adopt the same conformation that was found in the solid state.

Separation of peptides utilizing ultrahigh-temperature micellar electrokinetic chromatography

The use of ultrahigh column temperatures, up to 110 degrees C, in micellar electrokinetic capillary chromatography was investigated. The number of plates generated per unit time increased from 0.22 to 12.8 plates/s for separations at 15 degrees C and 110 degrees C, respectively. Ultrahigh-temperature micellar electrokinetic capillary chromatography was used for the separation of cyclic undecapeptides (cyclosporins). A minimum resolution of 1.39 was calculated for a critical peak pair at 110 degrees C, which is more than a 50% increase over resolution generated at 40 degrees C. During a run time of more than 90 min at 110 degrees C and at pH 9.3, no sample degradation or solvent boiling was observed.

Modification of cyclosporin A and conjugation of its derivative to HPMA copolymers

Cyclosporin A (CsA) was epoxidized with m-chloroperoxybenzoic acid in the presence of sodium carbonate or with tert-butyl hydroperoxide in the presence of dioxomolybdenum iminodiethanoxide. The CsA epoxide was not stable and rearranged into a compound with a more stable five-member ring structure. An amino group containing cyclosporin A derivative (CsA amine) was obtained by the reaction of CsA epoxide with excess ethylenediamine. The yield of the CsA amine was 30--40% based on the CsA. An HPMA copolymer--CsA conjugate was prepared by the reaction of the CsA amine with an HPMA and MA-Gly-Phe-Leu-Gly-ONp copolymer. The content of CsA amine in the conjugate was 8.7 wt %. The CsA amine was released from the copolymer by enzymatic hydrolysis with papain.

Pseudoprolines (psiPro) in drug design: direct insertion of psiPro systems into cyclosporin C

The insertion of acetals that exhibit variable structural features into complex peptides such as cyclosporin C (CsC) results in oxazolidine derivatives (pseudoprolines, psiPro) of tailored physico-chemical and biological properties. N,O-Acetalation of the 2-threonine hydroxyl group and the preceding amide nitrogen of CsC is achieved by treating the molecule with a number of both arylated and non-arylated dimethyl acetals. The psiPro-containing CsC derivatives exhibit enhanced conformational backbone rigidity, as suggested by analytical HPLC, NMR spectroscopy and by kinetic measurements on binding with their receptor protein cyclophilin A (CypA) that were not time-dependent. IC50 values for calf-thymus CypA were obtained by kinetic evaluation of its cis-->trans isomerase activity. The choice of the para-substituted aryl dimethyl acetals allows the inhibitory properties of the corresponding derivatives to be modulated to either prodrugs or moderately strongly binding cyclosporin C derivatives.

Technology evaluation: Valspodar, Novartis AG

Valspodar (PSC-833) is a derivative of cyclosporin but devoid of the immunosuppressive and nephrotoxic properties seen in cyclosporin A. It exhibited high affinity binding to Mdr1 P-glycoprotein (P-gp) and demonstrated multidrug resistance-reversing activity superior to cyclosporin A and verapamil both in vitro and in vivo. Preclinical and phase I/II clinical data have indicated that plasma levels of PSC-833 with multidrug resistance-reversing activities are achievable. Potent inhibition of intestinal, hepatobiliary and blood-brain barrier P-gp function has been demonstrated. Since valspodar is also a substrate of cytochrome P450 3A (CYP3A), dual interactions of this compound with P-gp and CYP3A are the basis for the pharmacokinetic interactions seen in preclinical and clinical studies. A new formulation of the drug has recently been developed with better oral bioavailability (60%) and less interindividual variability. The toxicity profiles of valspodar are acceptable and dose-limited by transient and reversible cerebellar ataxia. It has shown multidrug resistance-modulating activities towards acute myeloid leukemia, multiple myeloma and ovarian cancer in phase I/II clinical trials. Phase III studies with respect to these three diseases are ongoing.

Conformational polymorphism in peptidic and nonpeptidic drug molecules

Macrolide ligands that bind FK506 binding proteins and cyclosporins that a bind cyclophilins are chemically dissimilar but can share a number of structural and biological properties. Both families of ligands have very different conformations in the free state compared to those adopted when complexed with their binding protein. These transformations involve twisting from cis to trans about specific amide bonds, which result in significant changes in the hydrogen-bonding capabilities of the molecular surfaces. The three-dimensional structure of a new cyclosporin-like ligand (SDZ214 - 103) is described in the free crystalline state and bound to cyclophilin, and is shown to have a very different conformation from cyclosporin A in the free crystal, but a very similar conformation when bound to cyclophilin.

In vitro transepithelial drug transport by on-line measurement: cellular control of paracellular and transcellular transport

Studies on transcellular transport across epithelial cell layers are performed mostly by discontinuous sampling of the transported compound. This has several drawbacks, e.g., it gives disturbances in volume, it limits the time-resolution, and is often laborious. In this report we introduce a method to measure transepithelial transport of fluorescent compounds continuously. The time-resolution is at the (sub)minute scale, allowing the measurement of the change in transport rate before and after transport modulation. We will describe how we used the method to measure transcellular and paracellular transport. For highly membrane-impermeable compounds, the paracellular transport and the regulation of the tight junctions was studied in wild-type and MDR1 cDNA transfected epithelial canine kidney cells (MDCKII). The effect of the multidrug transporter P-glycoprotein (Pgp) on the transepithelial transport was studied. Addition of the Pgp inhibitor SDZ PSC 833 showed a modulation of the idarubicin (IDA) and daunorubicin (DNR) transport, which was larger during transport from the basolateral to the apical side than in the reverse direction. By modeling the transepithelial transport, we found that in these cells Pgp had more effect on the basolateral to apical transport than vice versa, which can be attributed to a relatively large passive permeation coefficient for the cellular basolateral plasma membrane.

Conformational differences of an immunosuppressant peptolide in a single crystal and in a crystal complex with human cyclophilin A

The crystal structure of (Thr2, Leu5, d-Hiv8, Leu10)-cyclosporin (cyclic peptolide SDZ 214-103) has been determined as the unbound crystal form and as a complex with human cyclophilin A. This pair of structures provides an example of a significant difference in conformation between free and bound ligand in crystals. The conformation of the unbound form is unlike that of both free and bound conformations of cyclosporin A (with the amide bond between residues 3 and 4 in the cis conformation), while the bound conformation is similar to that of CsA bound to cyclophilin. The cyclophilin-bound conformations of both ligands are similar, though this involves a significantly different waterellipsisligand hydrogen-bonding structure, which compensates for the chemical differences between the two ligands.

Isolation, identification and immunosuppressive activity of a new IMM-125 metabolite from human liver microsomes. Identification of its cyclophilin A-IMM-125 metabolite complex by nanospray tandem mass spectrometry

The isolation from human liver microsomes and identification by electrospray mass spectrometry and tandem mass spectrometry of a new metabolite of IMM-125 resulting from the biotransformation of the amino acid 1 vinylic methyl group to a carboxylic acid, called the IMM-125-COOH metabolite, is described. It was found that the complex of this new metabolite with cyclophilin A is formed less easily than the corresponding cyclophilin A-IMM-125-CH2OH main metabolite and cyclophilin A-IMM-125 complexes. However, when formed, the IMM-125-COOH metabolite-cyclophilin A complex requires more collision-induced dissociation (CID) to dissociate the complex than the complexes formed with the two other ligands. The nanospray tandem mass spectrum of the IMM-125-COOH metabolite-cyclophilin A complex (m/z 1755) gives rise to cyclophilin A-ligand complexes of m/z 1751 by elimination of CO2 and of m/z 1749 by loss of CO2 and H2O or glycerol. Since immunosuppressive activity is known to be dependent on the formation of a binary complex between cyclophilin A and the drug and since the target for the binary complex was found to be the calcium- and calmodulin-dependent protein phosphatase, calcineurin, it could be interesting to measure for structurally related immunosuppressive drugs the CID energy necessary to dissociate the binary complexes in order to evaluate whether a correlation with the phosphatase activity could be derived.

Applications of peptide synthetases in the synthesis of peptide analogues

Enzymatically formed peptides show positional variations as well as highly conserved amino acids. In the cases of gramicidin S, tyrocidine, linear gramicidins, enniatins, echinocandins and viridogrisein in vivo and in vitro studies indicate substrate selection at the level of amino acid activation as a major control step. Evidence for proof-reading steps beyond activation has been obtained in penicillin and cyclosporin biosynthesis. Activated substrate analogues may promote the formation of side products such as dipeptides and cyclodipeptides. Modifications of intermediates, such as N-methylation, influence the rates of peptide synthesis. These control steps pose limitations for the application of such enzyme systems in the production of peptide libraries. They may originate from a target oriented evolution of these synthetases.

Cyclosporin C is the main antifungal compound produced by Acremonium luzulae

A strain of Acremonium luzulae (Fuckel) W. Gams was selected in screening new microorganisms for biological control of fruit postharvest diseases, especially gray and blue mold diseases on apples and strawberries. This strain manifests a very strong activity against a large number of phytopathogenic fungi. In this work, the product responsible for this antifungal activity was isolated from modified Sabouraud dextrose broth cultures of A. luzulae. It was purified to homogeneity by reverse-phase column chromatography. On the basis of UV, infrared, and 1H and 13C nuclear magnetic resonance spectra, mass spectral analysis, and the amino acid composition of the acid hydrolysates, the antibiotic was determined to be cyclosporin C. Cyclosporin C showed a broad-spectrum activity against filamentous phytopathogenic fungi but no activity against bacteria or yeasts. Its antifungal activity is only fungistatic. In contrast to Tolypocladium inflatum, another cyclosporin-producing strain, A. luzulae, did not produce additional cyclosporins. This was confirmed by in vivo-directed biosynthesis.

New multidrug-resistance-reversing drugs, MS-209 and SDZ PSC 833

The emergence of multidrug resistance (MDR) is a major problem in cancer chemotherapy. Many compounds have been developed to reverse MDR, and some of them are undergoing clinical trials. Among them, MS-209, a novel quinoline derivative, is one of the most potent MDR-reversing agents: MS-209 at 3 microM effectively reverses MDR in various cell lines in vitro. MS-209 directly interacts with P-glycoprotein (Pgp) and inhibits Pgp-mediated drug transport. Oral administration of MS-209 combined with anticancer drugs significantly increases the life span of mice bearing MDR tumor cells without causing serious side effects. SDZ PSC 833, a non-immunosuppressive analogue of cyclosporin A (CsA), is another potent MDR-reversing drug. Interestingly, the MDR-reversing activity of SDZ PSC 833 is enhanced in vitro and in vivo by MRK-16, a monoclonal antibody that recognizes an extracellular epitope of Pgp. Since MRK-16 promotes immune responses to MDR tumor cells expressing Pgp, the combined use of MRK-16, SDZ PSC 833, and antitumor drugs could be an effective therapeutic modality to reverse MDR.

Cyclosporin analogs modified in the 3,7,8-positions: substituent effects on peptidylprolyl isomerase inhibition and immunosuppressive activity are nonadditive

Four analogs of cyclosporin A (CsA) were synthesized to determine if the biological activities of CsA analogs generated by multiple amino acid replacements are predictable from the effects on biological activity of analogs with single residue changes. CsA analogs [Phe7]CsA (8a), [D-MeAla3,Phe7]CsA (8b), [D-Ser8,Phe7]CsA (8c), and [D-MeAla3,Phe7,D-Ser8]CsA (8d) were designed by modification of positions 3, 7, and 8, which are adjacent to one effector region of the cyclophilin-bound CsA complex. The syntheses of CsA analogs 8a-d were carried out by suitable modifications of the reported strategy. Each analog was characterized by NMR in deuterated chloroform and DMSO solutions, and their biological activities as inhibitors of cis-trans-peptidyl prolyl isomerase (PPIase), inhibitors of proliferation in BDF1 mouse spleen cells stimulated with concanavalin A (Con A), and inhibitors of IL-2 release stimulated with PMA/ionomycin by Jurkat cells were determined. Incorporation of the phenylalanine residue in position 7 diminished activities 5-8-fold. Substitution at position 3 decreased activity nearly 2-fold, and substitution at position 8 did not lower activities. However, when all three modifications (D-MeAla3,Phe7, and D-Ser8) were incorporated into one molecule, the resulting analog, 8d, was found to bind more tightly to cyclophilin than CsA (Ki = 3 +/- 1.5 vs 6 +/- 2 nM) and to produce the full immunosuppressive effect in the other assay systems. Our structure-activity results show that combinations of substitutions that individually lower PPIase or immunosuppressive activity produce fully active analogs when combined in a single compound. These results suggest that other, multimodified CsA derivatives may be discovered that possess excellent or improved immunosuppressive activities even though they contain a substitution that otherwise reduces immunosuppressive activity.

Modeling conformational changes in cyclosporin A

NMR and X-ray structures for the immunosuppressant cyclosporin A (CsA) reveal a remarkable difference between the unbound (free) conformation in organic solvents and the conformation bound to cyclophilin. We have performed computer simulations of the molecular dynamics of CsA under a variety of conditions and confirmed the stability of these two conformations at room temperature in water and in vacuum. However, when the free conformation was modeled in vacuum at 600 K, a transition pathway leading to the bound conformation was observed. This involved a change in the cis MeLeu-9 peptide bond to a trans conformation and the movement of the side chains forming the dominant hydrophobic cluster (residues MeBmt-1, MeLeu-4, MeLeu-6, and MeLeu-10) to the opposite side of the plane formed by the backbone atoms in the molecular ring. The final conformation had a backbone RMS deviation from the bound conformation of 0.53 A and was as stable in dynamics simulations as the bound conformation. Our calculations allowed us to make a detailed analysis of a transition pathway between the free and the bound conformations of CsA and to identify two distinct regions of coordinated movement in CsA, both of which underwent transitions independently.

Inhibition of human immunodeficiency virus replication by nonimmunosuppressive analogs of cyclosporin A

Analogs of the immunosuppressive cyclic undecapeptide cyclosporin A (CsA) with substitutions in positions 1, 4, 6, and/or 11 were rationally designed to possess substantially diminished or no immunosuppressive activity. When these compounds were assayed for their capacity to interfere with the replication of human immunodeficiency virus, some displayed a potent antiviral activity in newly infected cells. However, only CsA could interfere with virus replication in persistently infected cells. One CsA analog with antiviral activity costimulated the phytohemagglutinin-induced production of interleukin 2 by human lymphocytes. Human immunodeficiency virus particles from drug-exposed cells showed lower infectivity than virions from untreated cells. Thus, these nonimmunosuppressive analogs of CsA constitute a promising class of lead compounds to develop drugs for effective treatment of the acquired immunodeficiency syndrome.

Sites of biotransformation for the cyclosporin derivative SDZ IMM 125 using human liver and kidney slices and intestine. Comparison with rat liver slices and cyclosporin A metabolism

SDZ IMM 125 (IMM), the hydroxyethyl derivative of cyclosporin A (CSA), is metabolized by human liver slices to analogous primary metabolites, hydroxylated IMM1 and IMM9 and N-demethylated IMM4N, as for CSA (M17/AM1, M1/AM9, and M21/AM4N), but the rate and extent of IMM biotransformation is less than for CSA. Initial rates of IMM metabolite formation in the human liver slice cultures are 6.6 +/- 2.8 nmol/hr/g liver at 1 microM IMM and 24.3 +/- 22.9 nmol/hr/g liver at 10 microM IMM, whereas the rate of CSA metabolite formation is 1.8-fold faster at both concentrations. The percentage of unchanged IMM is 73% at 1 microM and 80% at 10 microM after 24 hr, reflecting the lower extent of IMM metabolism, about one-third (1 microM) and one-half (10 microM) that of CSA. In rat liver slices, IMM is metabolized to the same primary metabolites as in human liver slices, but more slowly and remains 90% unchanged at 24 hr. Human jejunum formed the same primary metabolites of IMM and CSA as in liver. Upscaling the slice rate of biotransformation revealed that human jejunum would contribute considerably to the first-pass of IMM and CSA, being approximately 2 to 3-fold slower than the rate in liver. The inhibition of both IMM and CSA biotransformation by triacetyloleandomycin implicates the involvement of cytochrome P4503A proteins. Human kidney cortex slices metabolized IMM to IMM1 and IMM9, accounting for approximately 75% of the total metabolites. Total metabolite formation represented approximately 64% of liver metabolite formation.(ABSTRACT TRUNCATED AT 250 WORDS)

The importance of structural factors on the rate and the extent of N,O-acyl migration in cyclic and linear peptides

The chemistry associated with the process of N,O-acyl migration was explored in both cyclic and linear peptides under aqueous acid conditions. The importance of backbone cyclization and N-methylation of the peptide bond on the kinetics of N,O-acyl migration in a series of linear and cyclic peptides related in structure to cyclosporin A (CsA) were examined. The similarity in the chemical reactivity of the cyclic peptide [MeLeu (3-OH)]1-CsA and the corresponding linear peptide [Val-MeLeu (3-OH)-Abu], suggested that for this series, cyclization of the peptide backbone may not play an important role in controlling the kinetics of N,O-acyl migration. In contrast, the disparity in the chemical reactivity of tripeptides [Val-MeLeu (3-OH)-Abu] and [Val-Leu (3-OH)-Abu], indicated that N-methylation of amide bond significantly impacted the kinetics. Various hypothesis are proposed to account for this observation.

The cyclosporins

This review presents the progress and some aspects achieved during recent years with cyclosporin sources, chemistry, biological activities, side effects, biosynthesis and metabolism. Although incomplete the results indicate future research trends and some white spots to be studied in the near future to afford unique insights into cell biology and to improve the search for similar and even more specific agents based on rational drug design.

Hydrolysis of cyclosporin A: identification of 1,11 seco-cyclosporin A and 4,5 seco-isocyclosporin A by FAB-MS/MS

We have previously reported that treatment of CsA with aqueous HCI gives rise to the formation of a number of water-soluble compounds. Two of these were identified from their FAB-MS/MS spectra as open-chain nona- and decapeptides. We describe here the identification of two other main compounds deriving from the same treatment. Identification was rendered possible from the comparison of their FAB-MS/MS spectra with those of methyl and acetyl derivatives. The two compounds are water-soluble, open-chain undecapeptides corresponding to 1.11 seco-CsA and of 4.5 seco-isoCsA, respectively.

BF3-catalysed methanolysis combined with fast atom bombardment tandem mass spectrometry as a new tool for the preparation and analysis of linear secocyclosporins

BF3-catalysed methanolysis is presented for cyclic peptide cleavage using cyclosporins as model compounds. The reaction conditions for BF3-catalysed methanolysis of cyclosporins were optimized to favour the formation of linear undecapeptides. The resulting secocyclosporins were analysed by fast atom bombardment tandem mass spectrometry. Only one dominant and two side mechanisms of the ring opening are operating so that the complete sequence determination of all prepared oligopeptides was achieved.

Cyclosporins from Tolypocladium terricola

New natural cyclosporins were isolated from the mycelium of surface cultivated fungus Tolypocladium terricola. The chemical structures of [Leu4] CS and [MeLeu1] CS = cyclosporin-J, were deduced from the NMR and mass spectral data. Biological activity of new cyclosporins is reported based on the proliferative mitogen stimulation test.

The molecular replacement solution and X-ray refinement to 2.8 A of a decameric complex of human cyclophilin A with the immunosuppressive drug cyclosporin A

The X-ray structure of a decameric form of a complex of human cyclophilin A (CypA) with the immunosuppressive drug cyclosporin A (CsA) has been determined. The crystals of space group P43212 with cell dimensions a = b = 95.2 A, c = 280.0 A have five copies of the cyclophilin A/cyclosporin A complex in the asymmetric unit. The structure was solved by molecular replacement techniques, using a known cyclophilin A model. Procedures were developed to construct a self-rotation function using the results of cross-rotation searches. The comparison of experimental and constructed self-rotation maps was an important aid in selecting the correct rotation function solution. The translation functions revealed the presence of a cyclic pentamer. A crystallographic dimer axis passes through the non-crystallographic 5-fold rotation axis of the pentameric asymmetric unit, and generates a decameric "sandwich" of CypA/CsA heterodimers that has 52 symmetry. The five CypA/CsA protomers were refined independently using all data to 2.8 A giving a final crystallographic R-factor of 15.7%. Despite the constraints due to the packing arrangement within the decamer, the CypA and CsA conformations are similar to other CypA/CsA structures determined by X-ray crystallography and NMR spectroscopy. The hydrophobic CsA molecules are embedded in the middle of the decameric sandwich with only 20% of their surface exposed to solvent. The binding loop of CsA (residues 1 to 3 and 9 to 11) comprising 42% of the CsA surface, is buried in the peptidyl-prolyl-cis-trans isomerase active site of the cognate binding partner CypA, while the effector loop (residues 4 to 8) packs in the core of the decamer making hydrogen-bonding and van der Waals contacts with three neighbouring molecules. The environment of CsA in the decamer has been analysed and may provide a mimic for the interactions likely to occur between the CypA/CsA complex and its biological target calcineurin. There is no evidence to suggest that the decameric sandwich itself plays a role in immunosuppression by inhibiting calcineurin. However, the chaperone/foldase activity of CypA could require oligomer formation for its biological function.

Interaction of cyclosporin A and two cyclosporin analogs with cyclophilin: relationship between structure and binding

The immunosuppressant drug cyclosporin A exists as various conformers in water. Up to 1 h is needed to reach maximum complex formation after mixing the drug with its receptor, cyclophilin, or an antibody, indicating that only a fraction of the conformers in aqueous solution adopts a conformation suitable for binding. In the present study we compare the binding behavior of cyclosporin to that of two analogs, using a biosensor instrument (BIAcore, Pharmacia). The amount of complex formation was measured as a function of time after adding the peptides to cyclophilin. The equilibrium affinity constants of cyclophilin for these analogs have been measured. The slow binding of cyclosporin to cyclophilin compared to the instant binding of the cyclosporin analogs supports the hypothesis that cyclophilin recognizes a well defined conformation of cyclosporin that exists in water prior to binding.

Recent progress on regulation of the mitochondrial permeability transition pore; a cyclosporin-sensitive pore in the inner mitochondrial membrane

The mitochondrial permeability transition pore allows solutes with a m.w. approximately less than 1500 to equilibrate across the inner membrane. A closed pore is favored by cyclosporin A acting at a high-affinity site, which may be the matrix space cylophilin isozyme. Early results obtained with cyclosporin A analogs and metabolites support this hypothesis. Inhibition by cyclosporin does not appear to require inhibition of calcineurin activity; however, it may relate to inhibition of cyclophilin peptide bond isomerase activity. The permeability transition pore is strongly regulated by both the membrane potential (delta psi) and delta pH components of the mitochondrial protonmotive force. A voltage sensor which is influenced by the disulfide/sulhydryl state of vicinal sulfhydryls is proposed to render pore opening sensitive to delta psi. Early results indicate that this sensor is also responsive to membrane surface potential and/or to surface potential gradients. Histidine residues located on the matrix side of the inner membrane render the pore responsive to delta pH. The pore is also regulated by several ions and metabolites which act at sites that are interactive. There are many analogies between the systems which regulate the permeability transition pore and the NMDA receptor channel. These suggest structural similarities and that the permeability transition pore belongs to the family of ligand gated ion channels.

In vitro biosynthesis of ring-extended cyclosporins

Cyclosporin synthetase, a multifunctional polypeptide, catalyses the biosynthesis of the set of natural cyclosporins. We report that this enzyme is also capable of introducing a beta-alanine into position 7 or 8 of the ring instead of the alpha-alanines present at these positions in cyclosporin A. This leads to 34-membered rings in contrast to the 33-membered ring of the cyclo-undecapeptide cyclosporin A. Both [beta Ala7]CyA and [beta Ala8]CyA show immunosuppressive activity. The cyclosporin synthetase-related enzyme peptolide SDZ 214-103 synthetase, on the other hand, does not incorporate either beta-alanine into position 7 or beta-hydroxy acids into position 8, confirming the previously described higher substrate specificity of this enzyme compared with cyclosporin synthetase [Lawen and Traber (1993) J. Biol. Chem. 268, 20452-20465].

Conformational changes in the cyclic undecapeptide cyclosporin induced by interaction with metal ions. An FTIR study

Infra-red spectra have been measured for the cyclic undecapeptide cyclosporin A (CsA) and three analogues CsC, CsD and CsH in acetonitrile and in the presence of 10:1 molar excess of Mg2+, Ca2+, Na+ or Li+ in the same solvent. Interaction with each of these ions is suggested by marked changes in band positions over the amide I region (1600-1700 cm-1). The formation of complexes of cyclosporin with calcium and magnesium ions is indicated by the presence of C = O stretching bands well outside the range normally expected for the amide I absorptions of free peptides. Although they share this characteristic, the spectra indicate that the mode and/or strength of Ca2+ binding is quite different from that of Mg2+ binding. In contrast, the two monovalent ions interact with CsA, CsC and CsD to yield spectra that are very similar to one another. The spectra are consistent with binding of the monovalent ions simultaneously to several carbonyl groups of the loop structure.

The X-ray structure of (MeBm2t)1-cyclosporin complexed with cyclophilin A provides an explanation for its anomalously high immunosuppressive activity

For most of the cyclosporin A (CsA) analogs, there is generally a good correlation between cyclophilin binding and immunosuppression. However, this relationship does not seem to hold for 4-[(E)-2-butenyl]-4,4,N-trimethyl-L-threonine1 (MeBm2t)1-CsA. Its affinity for cyclophilin was reported to be approximately 1% that of CsA and its immunosuppressive activity in vitro was shown to be approximately 30% that of CsA. We report here the crystal structure of a complex between recombinant human cyclophilin A (CypA) and (MeBm2t)1-CsA which has been determined by X-ray crystallography at 2.2 A resolution and refined to an R-factor of 16.3%. (MeBm2t)1-CsA shows a similar bound conformation and network of interactions to CypA as CsA. The measured lower affinity for CypA cannot therefore be explained by a different mode of binding. We propose that the poor affinity to CypA could be accounted for by the existence of an equilibrium in aqueous solution between a 'cyclophilin bound conformation' and a 'non-binding conformation' of (MeBm2t)1-CsA. The relatively high immunosuppressive activity is suggested to result from slight conformational differences observed in the effector domain.

Kinetics of acid-catalyzed degradation of cyclosporin A and its analogs in aqueous solution

The kinetics and mechanism of the degradation of cyclosporin A have been studied under aqueous acidic conditions. The rate of degradation was found to be specific acid-catalyzed over the pH range studied (1-4), with isocyclosporin A as the predominant degradation product. Selective reduction of the olefinic bond of the amino acid 2-N-methyl-(R)-((E)-2-butenyl)-4-methyl-L-threonine (MeBmt) did not affect the overall degradation kinetics and product distribution of cyclosporin A. These observations indicate that the alternative degradation pathway involving intramolecular alkoxy addition to the olefinic bond of amino acid MeBmt apparently does not significantly contribute to the overall degradation kinetics of cyclosporin A in the pH range 1-4. The chemical reactivity of O-acetyl-cyclosporin A was examined to probe the governing mechanism for the isomerization of cyclosporin A. Under identical conditions, O-acetyl-cyclosporin A showed a much greater chemical stability than cyclosporin A, consistent with a mechanism involving the hydroxyoxazolidine intermediate. The chemical stability of cyclosporin C, which contains two beta-hydroxyl groups, was also examined. The rate and product distribution for the degradation of cyclosporin C suggest that under aqueous acidic conditions it undergoes N,O-acyl migration solely at the amino acid residue MeBmt. Additionally, the impact of side-chain bulkiness of amino acid MeBmt was examined by studying the degradation kinetics of a series of cyclosporin A analogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystal structures of cyclophilin A complexed with cyclosporin A and N-methyl-4-[(E)-2-butenyl]-4,4-dimethylthreonine cyclosporin A

BACKGROUND

Cyclophilin (CyP) is a ubiquitious intracellular protein that binds the immunosuppressive drug cyclosporin A (CsA). CyP-CsA forms a ternary complex with calcineurin and thereby inhibits T-cell activation. CyP also has enzymatic activity, catalyzing the cis-trans isomerization of peptidyl-prolyl amide bonds.

RESULTS

We have determined the structure of human cyclophilin A (CyPA) complexed with CsA to 2.1 A resolution. We also report here the structure of CyPA complexed with an analog of CsA, CsA (MeBm2t1-CsA), which binds less well to CyPA, but has increased immunosuppressive activity. Comparison of these structures with previously determined structures of unligated CyPA and CyPA complexed with a candidate substrate for the isomerase activity, the dipeptide AlaPro, reveals that subtle conformational changes occur in both CsA and CyPA on complex formation.

CONCLUSIONS

MeBm2t1-CsA binds to CyPA in an essentially similar manner to CsA. The 100-fold weaker affinity of its binding may be attributable to the close contact between MeBmt1 and the active site residue Ala103 of CyPA, which causes small conformational changes in both protein and drug. One change, the slight movement of MeLeu6 in CsA relative to MeBm2t1-CsA, may be at least partially responsible for the higher affinity of the CyPA-MeBm2t1-CsA complex for calcineurin. Our comparison between CyPA-CsA and CyPA-AlaPro suggests that CsA is probably not an analog of the natural substrate, confirming that the catalytic activity of CyPA is not related to its role in immunosuppression either structurally or functionally.