Cyclosporins. Structure-activity relationships

Cyclosporin A (Sandimmun) achieves immunosuppressive activity by complex formation with cyclophilin and subsequent binding of the binary complex to and inhibiting protein phosphatase 2B (calcineurin). Complexes of nonimmunosuppressive cyclophilin binding cyclosporin analogues do not inhibit protein phosphatase 2B, suggesting a crucial role for this enzyme in T cell activation. Binding of cyclosporin A to cyclophilins A, B, and C, respectively, results in complexes of significantly different inhibitory potency. The cyclosporin molecule thus has two functional domains, one mediating cyclophilin binding and a second one endowing affinity of the complex to calcineurin, thereby inhibiting its enzyme activity. Structure-activity studies and x-ray crystallography of cyclosporin-cyclophilin complexes indicate a crucial role of leucine side chains in positions 4 and 6 of the cyclosporin macrocycle for the calcineurin interaction.

Structure-based design of a cyclophilin-calcineurin bridging ligand

The affinity of a flexible ligand that adopts a specific conformation when bound to its receptor should be increased with the appropriate use of conformational restraints. By determining the structure of protein-ligand complexes, such restraints can in principle be designed into the bound ligand in a rational way. A tricyclic variant (TCsA) of the immunosuppressant cyclosporin A (CsA), which inhibits the proliferation of T lymphocytes by forming a cyclophilin-CsA-calcineurin complex, was designed with the known three-dimensional structure of a cyclophilin-CsA complex. The conformational restraints in TCsA appear to be responsible for its greater affinity for cyclophilin and calcineurin relative to CsA.

Substrate specificities of cyclosporin synthetase and peptolide SDZ 214-103 synthetase. Comparison of the substrate specificities of the related multifunctional polypeptides

The recently discovered multifunctional polypeptide cyclosporin synthetase is capable of synthesizing the cyclic undecapeptide cyclosporin A in a batch reaction. Substrates are the unmethylated constitutive amino acids of cyclosporin A. Exchange of one or more of these by various amino acids gives a picture of the substrate specificity of the enzyme in vitro, which is different from the known picture obtained by in vivo studies. The uncommon amino acid butenylmethylthreonine in position 1 of the cyclosporin ring can be exchanged by an unexpected large spectrum of different amino acids, showing a great flexibility of this site. Position 2, on the other hand, which shows the greatest variability in vivo, has an only slightly lower specificity in vitro. Position 3 has a very high degree of specificity; positions 4, 6, 7, 9, and 10 have marginally less. The variability of positions 5 and 11 is moderate, whereas position 8 shows only low substrate specificity in vitro. In general, most sites of SDZ 214-103 synthetase appear to be more specific than those of cyclosporin synthetase. Site 11 has nearly identical substrate specificity compared with that of cyclosporin synthetase. The D-2-hydroxy acid position (position 8) can be occupied by a large spectrum of substrates varying from D-lactic acid to D-2-hydroxyisocaproic acid. Within the limits of the present data, the addition of further functional groups to the D-2-hydroxy acid moiety are apparently not tolerated by the enzyme.

Peptidylproline cis-trans-isomerases: immunophilins

Two sequence-unrelated families of proteins possess peptidylproline cis-trans-isomerase activities (PPIase). PPIases are highly sequence conserved and multifunctional proteins which are present in many types of cells with a considerably divergent phylogenetic distribution. On the cellular level, PPIases occur in every compartment, both as free species and anchored to membranes. Diverse posttranslational modifications such as glycosylation, N-terminal modifications and phosphorylation constitute the additional functional features of PPIases. Folding, assembly and trafficking of proteins in the cellular milieu are regulated by PPIases. These enzymes accelerate the rate of in-vitro protein folding and they have the ability to bind proteins and act as chaperones. Some PPIases are coregulatory subunits of molecular complexes including heat-shock proteins, glucocorticoid receptors and ion channels. Secreted forms of PPIases are inflammatory and chemotactic agents for monocytes, eosinophils and basophils. The potent and clinically useful immunosuppressants CsA, FK506 or rapamycin bind with high affinities to PPIases (immunophilins). The binding criterion allows us to sort the PPIases for the following two superfamilies of proteins: the cyclophilins (CsA-binding proteins) and the FKBP (FK506/rapamycin-binding proteins). Although none of PPIases appeared to be essential for the viability of haploid yeast cells some of the immunophilin/immunosuppressant complexes are toxic both for yeast and mammalian cells. At least seven unlinked genes of cyclophilins and four unlinked genes of FKBP exist in human genomic DNA. Selected immunophilins regulate two different signalling pathways in lymphoid cells, namely the secretion of growth factors by stimulated T-cells and interleukin-2-induced T-cell proliferation. Moreover, selected FKBP mediate the cytotoxic effects of rapamycin in non-lymphoid cells. Accounts of the discovery of PPIases (immunophilins) and their functions are given in this review. A larger spectrum of proteins is analysed in relation to various signal-transduction pathways in lymphoid cells which involve immunophilins or their complexes with the immunosuppressants CsA, FK506 or rapamycin.

In vitro and in vivo comparative studies on immunosuppressive properties of cyclosporines A, C, D and metabolites M1, M17 and M21

Cyclosporine A (CsA) and its major metabolites: M1, M17 and M21 and two analogues: cyclosporines C (CsC) and D (CsD), were studied for their capacity to interfere with different in vitro activation pathways. Their inhibition potentials against the reaction of Graft-versus-Host (GvH) were also studied. The results showed: CsA, CsC and metabolite M17 were the most active compounds upon the inhibition of lymphocyte proliferation induced by different mitogens (ConA, PHA, PWM) and also on the proliferation of mixed lymphocyte cultures (MLC). The same results were observed concerning the direct activation by protein kinase C using a combined action of phorbol ester + calcium ionophore. In vivo using local GvH reaction, CsA and CsC proved more active than M17 in the two different combinations: H-2d --> (H-2b x H-2d)F1 and H-2k --> (H-2b x H-2k)F1 CsD and two metabolites M1 and M21 showed no or weak immunosuppressive effects. Overall, the immunosuppressive potency of six compounds could be schematized as: CsA > or = CsC > M17 > M1 > or = CsD > M21.

Solution structure of the cyclosporin A/cyclophilin complex by NMR

Cyclosporin A, a cyclic undecapeptide, is a potent immunosuppressant that binds to a peptidyl-prolyl cis-trans isomerase of 165 amino acids, cyclophilin. The cyclosporin A/cyclophilin complex inhibits the calcium- and calmodulin-dependent phosphatase, calcineurin, resulting in a failure to activate genes encoding interleukin-2 and other lymphokines. The three-dimensional structures of uncomplexed cyclophilin, a tetrapeptide/cyclophilin complex, and cyclosporin A when bound to cyclophilin have been reported. However, the structure of the cyclosporin A/cyclophilin complex has not been determined. Here we present the solution structure of the cyclosporin A/cyclophilin complex obtained by heteronuclear three-dimensional NMR spectroscopy. The structure, one of the largest determined by NMR, differs from proposed models of the complex and is analysed in terms of the binding interactions and structure/activity relationships for CsA analogues.

New cyclosporin A analogue: synthesis and immunosuppressive activity

A synthetic analogue of cyclosporine A, in which an unusual amino acid (4R)-N-methyl-4-butenyl-4-methyl-L-threonine (MeBmt) is replaced with L-threonine (Thr), was synthesized by the solid phase method. Its activity in the humoral response to sheep red blood cells in vitro and in vivo in mice was practically the same as that of cyclosporine A used as a standard, whereas the analogue studied exerted a significantly stronger effect in the delayed type hypersensitivity to sheep red blood cells in mice.

Kinetics and mechanism of isomerization of cyclosporin A

The kinetics of isomerization of cyclosporin A to isocyclosporin A were studied in various nonaqueous solvents as a function of temperature and added methanesulfonic acid. The rate of isomerization was found to be acid-catalyzed over the acid concentration range studied. The choice of organic solvent significantly altered the rate of isomerization. For a series of alcohols, the rate was enhanced with increasing dielectric constant of the media, however, this correlation did not hold upon introduction of the dipolar aprotic solvent, tetrahydrofuran. Conversion of cyclosporin A to isocyclosporin A in tetrahydrofuran was found to contain diminished side reactions as compared to alcoholic solvents. The rate of conversion of isocyclosporin A to cyclosporin A was determined in aqueous buffers as a function of pH, buffer concentration, and temperature. The rates of conversion were extremely rapid compared to the forward reaction. Based on the pH dependencies of dilute solution reactivities, isocyclosporin A displayed a kinetically generated pKa value of 6.9 for the secondary amine moiety. From pH 8 to pH 10 the pH-rate profile plot is linear, with a slope approximately equal to unity, indicating apparent hydroxide ion catalysis. The break in pH-rate profile suggests a change in the rate-determining step upon protonation of isocyclosporin A. The rate of isomerization in plasma was comparable with that found in a pH 7.4 buffer solution, indicating that plasma proteins do not significantly alter the isomerization kinetics of isocyclosporin A to cyclosporin A.

Inhibition of T cell signaling by immunophilin-ligand complexes correlates with loss of calcineurin phosphatase activity

Calcineurin, a Ca2+, calmodulin-dependent protein phosphatase, was recently found to bind with high affinity to two different immunosuppressant binding proteins (immunophilins) with absolute dependence on the presence of the immunosuppressants FK506 or cyclosporin A (CsA) [Liu et al. (1991) Cell 66, 807-815]. The binding affinities of the immunophilin-drug complexes toward calcineurin and the stoichiometry of the resultant multimeric complexes have now been determined, and structural elements of FK506, CsA, and calcineurin that are critical for mediating their interactions have been identified. Analogues of FK506 (FK520, FK523, 15-O-demethyl-FK520) and CsA (MeBm2t1-CsA and MeAla6-CsA) whose affinities for their cognate immunophilins do not correlate with their immunosuppressive activities have been prepared and evaluated in biochemical and cellular assays. We demonstrate a strong correlation between the ability of these analogues, when bound to their immunophilins, to inhibit the phosphatase activity of calcineurin and their ability to inhibit transcriptional activation by NF-AT, a T cell specific transcription factor that regulates IL-2 gene synthesis in human T cells. In addition, FKBP-FK506 and CyP-CsA do not inhibit members of the PP1, PP2A, and PP2C classes of serine/threonine phosphatases. These data suggest that calcineurin is the relevant cellular target of these immunosuppressive agents and is involved in Ca(2+)-dependent signal transduction pathways in, among others, T cells and mast cells.

The X-ray structure of a tetrapeptide bound to the active site of human cyclophilin A

Human cyclophilin A (165 residues) has peptidyl-prolyl cis-trans isomerase activity. Here we report a high-resolution three-dimensional X-ray structure of a substrate, ac-Ala-Ala-Pro-Ala-amc (ac, acetyl; amc, amidomethylcoumarin) bound to the active-site of cyclophilin. The structure consisting of a dimer of complexes and 135 water molecules was refined to a crystallographic R-factor of 17.7% for all data in the range 8 A-2.3 A.

Cyclosporins as drug resistance modifiers

Cyclosporin A (CsA), a cyclic peptide of 11 amino acids isolated from the fungus Tolypoclodium inflatum Gams, is the principle drug used for immunosuppression in organ transplant patients. It is known to have a very specific effect on T-cell proliferation although the precise mechanism remains unclear. Following internalization, CsA binds to a cytosolic protein, cyclophilin, which has been shown to possess peptidyl-prolyl cis-trans isomerase activity. CsA is an effective modifier of multidrug resistance in human and rodent cells at doses in the range of 1 to 5 micrograms/mL. Although it reverses the drug accumulation deficit associated with multidrug resistance in some cell types, this is not always the case. CsA has P-glycoprotein binding activity but less specific membrane effects and inhibition of protein kinase C may also be involved in its resistance modifier action. A number of non-immunosuppressive analogues of CsA have been shown to have resistance modifier activity and some are more potent than the parent compound. One analogue from Sandoz, PSC-833, has been shown to be approximately 10-fold more potent than CsA and is expected to enter clinical trial in the near future. The use of such agents may allow a full test of the hypothesis that reversal of multidrug resistance will prove a useful clinical strategy.

Analogs of cyclosporin A modified at the D-Ala8 position

The conversion of [2-deutero-3-fluoro-D-Ala8]cyclosporin A (1) to a dehydroalanine analog [delta-Ala8]cyclosporin A (2) was achieved with lithium diisopropylamide in THF at low temperature. This dehydro compound is a useful intermediate for the preparation of position 8 analogs of cyclosporin A formed from it by the conjugate addition of thiol compounds. NMR conformational studies have provided evidence for the restoration of D-stereochemistry in the modified Ala8 residues. The preparation of several of these cyclosporin analogs and their bioactivities are described.

Preparation of novel cyclosporin A derivatives

The hydroxyl group on the 2-N-methyl-(R)-((E)-2-butenyl)-4-methyl-L-threonine residue of cyclosporin A was protected by acetylation, then the double bond on the same amino acid residue was oxidatively cleaved using a periodate/permanganate reagent. The resultant derivative of cyclosporin A contained a carboxylic acid group which was subsequently reacted with the nucleophiles 5-(aminoacetamido)fluorescein and poly(L-lysine), in the presence of 1-ethyl-3-[3-(dimethylamino)propyl] carbodiimide, to furnish novel cyclosporin A conjugates.

New fluorescent derivatives of cyclosporin for use in immunoassays

The synthesis of new fluorescent derivatives of cyclosporin is described and their affinity with the specific Sandoz monoclonal antibody investigated. Synthesis was carried out using cyclosporin-C-hemisuccinate as the starting material with monodansylcadaverine, 4-bromomethyl-7-methoxycoumarin, and 4-bromomethyl-6,7-dimethoxycoumarin as labels. After extraction the derivatives were purified by HPLC and their binding affinity with the monoclonal antibody evaluated by the Incstar Cyclo-Trac SP radioimmunoassay. All three derivatives showed good binding and it is suggested that they may be of use in an immunoassay for measuring cyclosporin.

The nucleotide sequence of a third cyclophilin-homologous gene from Saccharomyces cerevisiae

The nucleotide sequence of a 1558 bp DNA fragment from the right arm of chromosome III of Saccharomyces cerevisiae contains an open reading frame of 954 nucleotides with coding potential for a protein with high similarity to the ubiquitous cyclophilins which are both peptidyl-prolyl cis-trans isomerases and cyclosporin A-binding proteins. It should, therefore, represent the third gene (SCC3) of this kind from S. cerevisiae. SCC3 is present in a single copy in the genome of S. cerevisiae and results in a constitutively expressed 1.2 kb transcript during cell growth. Its putative protein product (Scc3) contains two hydrophobic cores, one at the amino terminal, 20 amino acids long, which could serve as a signal peptide, and the other one at the carboxyl end with a structure similar to a transmembrane helix. These findings suggest that Scc3 could be a secretory or, more likely, a transmembrane protein. The only cyclophilin with similar structure to that of Scc3 is ninaA from Drosophila melanogaster, a transmembrane protein which seems to be implicated in the correct folding and/or intercalation of rhodopsin in the endoplasmic reticulum of the fly photoreceptors (Stamnes, M.A. et al., Cell 65, 219-227, 1991). In addition, the amino and the carboxy regions of Scc3 and ninaA share a significant level of homology, which suggests that they have a similar function, albeit for different target proteins.

Formation of cyclosporins by Tolypocladium inflatum

Submerged cultures of the low-production strain of Tolypocladium inflatum DSM 63544 formed a mixture of cyclosporins (CS) consisting of CS-A, CS-B, "CS-3" and "CS-4". Glucose, sucrose and maltose were highly favored for biomass production but provided a different physiological state necessary for CS biosynthesis. Not only the magnitude of CS production but also the proportion of individual components of the CS mixture were affected by the C source. Intensive CS synthesis was in correlation with the formation of CS-3. Lower yields of CS were accompanied by an increased proportion of CS-A in the CS complex. The best specific production of CS was achieved on the glucose medium, the highest yield of CS-A on the maltose medium. There was no remarkable relationship between the biomass formation and the intensity of CS synthesis.

Selective 13C-labelling of cyclosporin A

Cyclosporin A is biosynthetically labelled with 13C by growing an overproducing strain of Tolypocladium inflatum on minimal media containing either [1-13C]-, [2-13C]-, [3-13C]- or [6-13C]glucose as the only carbon source. NMR analysis of the 13C-labelled peptide showed a labelling pattern in which 13C occurs at specific sites. These can be predicted by consideration of the relevant biosynthetic pathways. Quantitation of the site-specific enrichments revealed that the 13C-label incorporation is efficient and selective. Metabolic fluxes through alternative pathways can also be estimated from these results. Isotopically labelled peptides will be a very useful tool for the study of molecular interactions with their receptors.

NMR studies of [U-13C]cyclosporin A bound to cyclophilin: bound conformation and portions of cyclosporin involved in binding

Cyclosporin A (CsA), a potent immunosuppressant, is known to bind with high specificity to cyclophilin (CyP), a 17.7 kDa protein with peptidyl-prolyl isomerase activity. In order to investigate the three-dimensional structure of the CsA/CyP complex, we have applied a variety of multidimensional NMR methods in the study of uniformly 13C-labeled CsA bound to cyclophilin. The 1H and 13C NMR signals of cyclosporin A in the bound state have been assigned, and from a quantitative interpretation of the 3D NOE data, the bound conformation of CsA has been determined. Three-dimensional structures of CsA calculated from the NOE data by using a distance geometry/simulated appealing protocol were found to be very different from previously determined crystalline and solution conformations of uncomplexed CsA. In addition, from CsA/CyP NOEs, the portions of CsA that interact with cyclophilin were identified. For the most part, those CsA residues with NOEs to cyclophilin were the same residues important for cyclophilin binding and immunosuppressive activity as determined from structure/activity relationships. The structural information derived in this study together with the known structure/activity relationships for CsA analogues may prove useful in the design of improved immunosuppressants. Moreover, the approach that is described for obtaining the structural information is widely applicable to the study of small molecule/large molecule interactions.

The NMR structure of cyclosporin A bound to cyclophilin in aqueous solution

Cyclosporin A bound to the presumed receptor protein cyclophilin was studied in aqueous solution at pH 6.0 by nuclear magnetic resonance spectroscopy using uniform 15N- or 13C-labeling of cyclosporin A and heteronuclear spectral editing techniques. Sequence-specific assignments were obtained for all but one of the cyclosporin A proton resonances. With an input of 108 intramolecular NOEs and four vicinal 3JHN alpha coupling constants, the three-dimensional structure of cyclosporin A bound to cyclophilin was calculated with the distance geometry program DISMAN, and the structures resulting from 181 converged calculations were energy refined with the program FANTOM. A group of 120 conformers was selected on the basis of the residual constraint violations and energy criteria to represent the solution structure. The average of the pairwise root-mean-square distances calculated for the backbone atoms of the 120 structures was 0.58 A. The structure represents a novel conformation of cyclosporin A, for which the backbone conformation is significantly different from the previously reported structures in single crystals and in chloroform solution. The structure has all peptide bonds in the trans form, contains no elements of regular secondary structure and no intramolecular hydrogen bonds, and exposes nearly all polar groups to its environment. The root-mean-square distance between the backbone atoms of the crystal structure of cyclosporin A and the mean of the 120 conformers representing the NMR structure of cyclosporin A bound to cyclophilin is 2.5 A.

Micro-quantification of cyclosporine and its metabolites and determination of their spectral absorptivities

We report a micromethod for the analysis of cyclosporine (CsA), based on quantification of its constituent amino acids. The amino acids were released by gas-phase hydrolysis, derivatized with fluorenylmethyl chloroformate, and separated and analyzed in a reversed-phase HPLC system. The imprecision (CV) of the amino acid analysis was less than 4%, and several determinations of the amount of standard CsA were within 1% of the weighed material. The detection limits (signal-to-noise ratio = 2) were 500 fmol for ultraviolet detection and 100 fmol for fluorescence detection. We also used this method to determine the ultraviolet absorptivities of CsA and five metabolites at 210, 214, and 230 nm. The molar absorptivity of most metabolites was about 10% higher than that of CsA, although the metabolite that was oxidized to a carboxyl group on the terminal carbon of N-methyl-butenyl-methyl-threonine (AM1A) had a molar absorptivity about 40% higher than that of CsA.

The isolation, structural characterization, and immunosuppressive activity of cyclosporin G (NVa2-cyclosporine) metabolites

Seven cyclosporin G metabolites were isolated by high-performance liquid chromatography from the urine of normal subjects receiving the drug. The structure and purity of the metabolites were assessed by fast atom bombardment/mass spectroscopy, by proton nuclear magnetic resonance (NMR), and by 13C-NMR. The structural modifications of the cyclosporin G metabolites consisted primarily of hydroxylation and demethylation, as is the case for cyclosporin A metabolites. The immunosuppressive activities of the metabolites were tested in three separate in vitro systems: a primary and secondary mixed lymphocyte system, as well as a mitogen stimulated system. In general, the metabolites have immunosuppressive activity of less than 10% of cyclosporin G. The significance of these findings in relation to the therapeutic monitoring of cyclosporin G is discussed.

Proton and nitrogen sequential assignments and secondary structure determination of the human FK506 and rapamycin binding protein

Sequential 1H and 15N assignments of human FKBP, a cytosolic binding protein for the immunosuppressive agents FK506 and rapamycin, are reported. A combination of homonuclear and relayed heteronuclear experiments has enabled assignment of 98 of 99 backbone amide NHs, 119 of 120 C alpha Hs, 97 of 99 non-proline amide 15Ns, and 375 of 412 side-chain resonances of this 107-residue protein. Long-range NOEs are used to demonstrate that FKBP has a novel folding topology consisting of a five-stranded antiparallel beta sheet with +3, +1, -3, +1 loop connectivity.

Conformation of MeAla6-cyclosporin A by NMR. Relationship of sidechain orientation of the MeBmt-1, MeLeu-9, and MeLeu-10 residues to immunosuppressive activity

MeAla6-cyclosporin A (MeAla6-CsA) is a unique CsA analog that shows weak immunosuppressive activity and yet binds strongly to the proposed cytosolic protein receptor, cyclophilin (CyP). Preliminary 1H NMR data showed significant chemical shift differences between spectra of MeAla6-CsA and CsA, suggesting different preferred conformations. A more detailed study, however, revealed that the backbone conformations of the two molecules are essentially identical, and that the differences can be accounted for, principally, by the sidechain motions of the MeBmt-1, MeLeu-9, and -10 residues. ROE and coupling constant data show that in MeAla6-CsA, the preferred chi 1 rotamers for MeLeu-9 and -10 are + 180 degrees (T), whereas in CsA there is a more even distribution of rotamer populations for MeLeu-10, and a preferred -60 degrees (G-) chi 1 rotamer for MeLeu-9. Similar data argue that the sidechain of MeBmt-1 is more restricted in its motion in MeAla-CsA than in CsA. Temperature studies suggest that these preferred rotamers for MeAla6-CsA may increase the stability of the hydrogen bond between NH(7) and CO(11), but prevent particular residues, especially the essential MeBmt-1 sidechain, from adopting orientations required to elicit immunosuppressive activity. The significant changes observed in the preferred orientations for the sidechains of the MeBmt-1, MeLeu-9, and MeLeu-10 residues in MeAla6-CsA argue that the particular orientations which they assume in CsA are not essential for cyclophilin binding.

Unusual solubility behaviour of cyclosporin A in aqueous media

The solubility of cyclosporin A was determined in water and in Sorensen buffers at pH 1.2 and 6.6 at temperatures ranging from 5 to 37 degrees C. No differences in solubility behaviour were observed among the three aqueous media. Solubility was found to be inversely proportional to the temperature in each medium, indicating that the heat of solution was exothermic in each case.

The clinical significance of cyclosporine metabolites

Cyclosporine (CsA) is extensively metabolized, with over 14 metabolites having been characterized to date. The confirmation of structure and purity is a prerequisite for studies involving CsA metabolites. Analytical techniques such as fast atom bombardment/mass spectroscopy (FAB/MS), tandem mass spectrometry (MS), 1H- and 13C-nuclear magnetic resonance (NMR) can be used for such purposes. In vitro experiments indicate that metabolites are considerably less immunosuppressive and toxic than CsA. In vivo studies have been hampered by sufficient quantities of metabolites and a suitable animal model. Preliminary results in the rat suggest that CsA metabolites are less immunosuppressive and toxic than CsA, although these results must be confirmed using a more suitable animal model. Present data indicate that the routine monitoring of metabolites is not warranted in transplant patients, although additional information is required to confirm these findings.

The immunosuppressive activity of the aldehydic transformation of cyclosporine on alloreactive T-cells

Cyclosporine (CsA) is a potent immunosuppressive compound, and its metabolites have previously been shown to have pharmacologic activity. The aldehydic metabolites have been isolated and are a metabolic intermediate after the conversion of CsA to its most active hydroxylated metabolite. The in vitro sensitivity of alloreactive T-lymphocytes, which are generated from a mixed lymphocyte reaction and propagated from organ transplant biopsy specimens to the aldehydic metabolites of CsA, was tested. In secondary proliferative assays in the presence of varying concentrations of CsA and the aldehydes, the concentration required to inhibit proliferation by 50% was 50 to 150 ng/mL for CsA and 3150 to 3500 ng/mL for the aldehydes. Pretreatment of alloreactive cells with CsA or the aldehydes did not alter cell viability, as tested with dye exclusion, or cell reactivity on reculturing. These studies concluded that the structural modification formed by metabolism of CsA to the aldehydic structure eliminates its antiproliferative activity on T-lymphocytes.

A method for preparing biologically active aqueous cyclosporin A solutions avoiding the use of detergents or organic solvents

The immunosuppressive agent cyclosporin A (CsA) is highly hydrophobic and is, therefore, being taken for in vitro use directly from ethanolic stock solutions. In the present study it is demonstrated that ethanol itself affects several immunological activities in vitro, thus interfering with the immunosuppressive effects of CsA. As an alternative, it is demonstrated that CsA can be solubilized and retain its activity in aqueous solutions of polyvinyl pyrrolidone (PVP). Unlike ethanol, PVP has no effect on the various immunological activities in vitro and therefore the immunosuppressive effects of CsA can be evaluated without interference.

Isotope-edited NMR of cyclosporin A bound to cyclophilin: evidence for a trans 9,10 amide bond

The binding of a 13C-labeled cyclosporin A (CsA) analog to cyclophilin (peptidyl prolyl isomerase) was examined by means of isotope-edited nuclear magnetic resonance (NMR) techniques. A trans 9,10 peptide bond was adopted when CsA was bound to cyclophilin, in contrast to the cis 9,10 peptide bond found in the crystalline and solution conformations of CsA. Furthermore, nuclear Overhauser effects (NOEs) were observed between the zeta 3 and epsilon 3 protons of the methylleucine (MeLeu) residue at position 9 of CsA and tryptophan121 (Trp121) and phenylalanine (Phe) protons of cyclophilin, suggesting that the MeLeu9 residue of CsA interacts with cyclophilin. These results illustrate the power of isotope-edited NMR techniques for rapidly providing useful information about the conformations and active site environment of inhibitors bound to their target enzymes.

Assessment of variables contributing to cyclosporine distribution in blood

Factors which can account for the poor correlation between whole blood and plasma Cyclosporine (CsA) levels in patients on CsA prophylaxis are evaluated. The study took account of the influence of plasma separation procedures, and the sample haematocrit on CsA distribution in the blood of renal transplant patients (n = 35). CsA was measured using both specific and non-specific CsA radioimmunoassays. Significant negative correlations occurred between CsA distribution and the haematocrit, independently of the plasma separation procedure or the specificity of the assay. All results were lower when using the specific assay but a significantly higher percentage of CsA was measured in the plasma by specific assay compared to nonspecific assay when plasma was separated at both 22 degrees C (t-test, p less than 0.02) and at 37 degrees C, p less than 0.01). This may relate to the selective binding of CsA and its analogues by blood cells. This study is a prelude to the development of more consistent plasma separation procedures in the monitoring of this drug.