Optimization of 2-Amino-4,6-diarylpyrimidine-5-carbonitriles as Potent and Selective A1 Antagonists.

We herein document a large collection of 108 2-amino-4,6-disubstituted-pyrimidine derivatives as potent, structurally simple, and highly selective A1AR ligands. The most attractive ligands were confirmed as antagonists of the canonical cyclic adenosine monophosphate pathway, and some pharmacokinetic parameters were preliminarilly evaluated. The library, built through a reliable and efficient three-component reaction, comprehensively explored the chemical space allowing the identification of the most prominent features of the structure-activity and structure-selectivity relationships around this scaffold. These included the influence on the selectivity profile of the aromatic residues at positions R4 and R6 of the pyrimidine core but most importantly the prominent role to the unprecedented A1AR selectivity profile exerted by the methyl group introduced at the exocyclic amino group. The structure-activity relationship trends on both A1 and A2AARs were conveniently interpreted with rigorous free energy perturbation simulations, which started from the receptor-driven docking model that guided the design of these series.

C2-substituted quinazolinone derivatives exhibit A1 and/or A2A adenosine receptor affinities in the low micromolar range.

Antagonists of the adenosine receptors (A1 and A2A subtypes) are widely researched as potential drug candidates for their role in Parkinson's disease-related cognitive deficits (A1 subtype), motor dysfunction (A2A subtype) and to exhibit neuroprotective properties (A2A subtype). Previously the benzo-α-pyrone based derivative, 3-phenyl-1H-2-benzopyran-1-one, was found to display both A1 and A2A adenosine receptor affinity in the low micromolar range. Prompted by this, the α-pyrone core was structurally modified to explore related benzoxazinone and quinazolinone homologues previously unknown as adenosine receptor antagonists. Overall, the C2-substituted quinazolinone analogues displayed superior A1 and A2A adenosine receptor affinity over their C2-substituted benzoxazinone homologues. The benzoxazinones were devoid of A2A adenosine receptor binding, with only two compounds displaying A1 adenosine receptor affinity. In turn, the quinazolinones displayed varying degrees of affinity (low micromolar range) towards the A1 and A2A adenosine receptor subtypes. The highest A1 adenosine receptor affinity and selectivity were favoured by methyl para-substitution of phenyl ring B (A1Ki = 2.50 µM). On the other hand, 3,4-dimethoxy substitution of phenyl ring B afforded the best A2A adenosine receptor binding (A2AKi = 2.81 µM) among the quinazolinones investigated. In conclusion, the quinazolinones are ideal lead compounds for further structural optimization to gain improved adenosine receptor affinity, which may find therapeutic relevance in Parkinson's disease-associated cognitive deficits and motor dysfunctions as well as exerting neuroprotective properties.

New Insights into Key Determinants for Adenosine 1 Receptor Antagonists Selectivity Using Supervised Molecular Dynamics Simulations.

Adenosine receptors (ARs), like many otherGprotein-coupledreceptors (GPCRs), are targets of primary interest indrug design. However, one of the main limits for the development of drugs for this class of GPCRs is the complex selectivity profile usually displayed by ligands. Numerous efforts have been madefor clarifying the selectivity of ARs, leading to the development of many ligand-based models. The structure of the AR subtype A1 (A1AR) has been recently solved,providing important structural insights. In the present work, we rationalized the selectivity profile of two selective A1AR and A2AAR antagonists, investigating their recognition trajectories obtained by Supervised Molecular Dynamics from an unbound state and monitoring the role of the water molecules in the binding site.

A Taxicab geometry quantification system to evaluate the performance of in silico methods: a case study on adenosine receptors ligands.

Among still comparatively few G protein-coupled receptors, the adenosine A2A receptor has been co-crystallized with several ligands, agonists as well as antagonists. It can thus serve as a template with a well-described orthosteric ligand binding region for adenosine receptors. As not all subtypes have been crystallized yet, and in order to investigate the usability of homology models in this context, multiple adenosine A1 receptor (A1AR) homology models had been previously obtained and a library of lead-like compounds had been docked. As a result, a number of potent and one selective ligand toward the intended target have been identified. However, in in vitro experimental verification studies, many ligands also bound to the A2AAR and the A3AR subtypes. In this work we asked the question whether a classification of the ligands according to their selectivity was possible based on docking scores. Therefore, we built an A3AR homology model and docked all previously found ligands to all three receptor subtypes. As a metric, we employed an in vitro/in silico selectivity ranking system based on taxicab geometry and obtained a classification model with reasonable separation. In the next step, the method was validated with an external library of, selective ligands with similarly good performance. This classification system might also be useful in further screens.

Synthesis and evaluation of methoxy substituted 2-benzoyl-1-benzofuran derivatives as lead compounds for the development adenosine A1 and/or A2A receptor antagonists.

A series of fourteen methoxy substituted 2-benzoyl-1-benzofuran derivatives were synthesised and their affinities determined for adenosine A1 and A2A receptors via radioligand binding assays to establish the structure activity relationships pertinent for A1 and A2A affinity. Compound 3j (6,7-dimethoxybenzofuran-2-yl)(3-methoxyphenyl)methanone exhibited A1 affinity (A1Ki (rat) = 6.880 µM) as well as A2A affinity (A2AKi (rat) = 0.5161 µM). Compounds 3a-b &3i-k exhibited selective affinity towards A1 with Ki values below 10 µM. The results indicate that C6,7-diOCH3 substitution on ring A in combination with meta (C3')-OCH3 substitution on ring B is beneficial for A1 and A2A affinity and activity. Compounds 3a-b &3j-k showed low cytotoxicity. Upon in vitro and in silico evaluation, compound 3j may be considered lead-like (i.e. a molecular entity suitable for optimization) and, thus, of value in the design of novel, potent and selective adenosine A1 and A2A receptor antagonists.

A newly synthesized 6-methyl-7H,8H,9H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin-8-one for potential inhibitor of adenosine A1 receptor: a combined experimental and computational studies.

6-Methyl-7H,8H,9H-[1,2,4]triazolo[4,3-b][1,2,4]triazepin-8-onehas been synthesized, characterized by spectroscopic techniques (FT-IR, 1H and 13C NMR) and finally the structure was confirmed by single crystal X-ray diffraction studies. In the title molecule, C6H7N5O, the 7-membered ring adopts a bowl-like conformation. In the crystal, the molecules form stacks along the c-axis direction through offset π-stacking interactions between the 5-membered rings and C-H···N hydrogen bonds. The stacks are associated via a combination of N-H···N, C-H···O and C-H···N hydrogen bonds. Further, the Hirshfeld surface analysis reveals the nature of molecular interactions and the fingerprint plot provides information about the percentage contribution from each individual molecular contact to the surface. In addition, due to its biological interest the target molecule adenosine A1 receptor was found based on Structural Activity Relationship (SAR) analysis and, further, subjected into molecular docking and molecular dynamics analysis to understand the binding interaction and stability of the molecule in adenosine A1 receptor system. Furthermore, the Density Functional Theory (DFT) calculations were carried for free compound and the compound in active site (single point DFT), to know the internal stability.Communicated by Ramaswamy H. Sarma.

An Advanced In Silico Modelling of the Interaction between FSCPX, an Irreversible A1 Adenosine Receptor Antagonist, and NBTI, a Nucleoside Transport Inhibitor, in the Guinea Pig Atrium.

In earlier studies, we generated concentration-response (E/c) curves with CPA (N6-cyclopentyladenosine; a selective A1 adenosine receptor agonist) or adenosine, in the presence or absence of S-(2-hydroxy-5-nitrobenzyl)-6-thioinosine (NBTI, a selective nucleoside transport inhibitor), and with or without a pretreatment with 8-cyclopentyl-N3-[3-(4-(fluorosulfonyl)-benzoyloxy)propyl]-N1-propylxanthine (FSCPX, a chemical known as a selective, irreversible A1 adenosine receptor antagonist), in isolated, paced guinea pig left atria. Meanwhile, we observed a paradoxical phenomenon, i.e. the co-treatment with FSCPX and NBTI appeared to enhance the direct negative inotropic response to adenosine. In the present in silico study, we aimed to reproduce eight of these E/c curves. Four models (and two additional variants of the last model) were constructed, each one representing a set of assumptions, in order to find the model exhibiting the best fit to the ex vivo data, and to gain insight into the paradoxical phenomenon in question. We have obtained in silico evidence for an interference between effects of FSCPX and NBTI upon our ex vivo experimental setting. Regarding the mechanism of this interference, in silico evidence has been gained for the assumption that FSCPX inhibits the effect of NBTI on the level of endogenous (but not exogenous) adenosine. As an explanation, it may be hypothesized that FSCPX inhibits an enzyme participating in the interstitial adenosine formation. In addition, our results suggest that NBTI does not stop the inward adenosine flux in the guinea pig atrium completely.

2-Benzylidene-1-Indanone Analogues as Dual Adenosine A1/A2a Receptor Antagonists for the Potential Treatment of Neurological Conditions.

Previous studies explored 2-benzylidine-1-tetralone derivatives as innovative adenosine A1 and A2A receptor antagonists for alternative non-dopaminergic treatment of Parkinson's disease. This study's aim is to investigate structurally related 2-benzylidene-1-indanones with substitutions on ring A and B as novel, potent and selective adenosine A1 and A2A receptor blockers. 2-Benzylidene-1-indanone derivatives were synthesised via acid catalysed aldol condensation reactions and evaluated via radioligand binding assays to ascertain structure activity relationships to govern A1 and A2A AR affinity. The results indicated that hydroxy substitution at C4 of ring A and meta (3'), or para (4') substitution on ring B of the 2-benzylidene-1-indanone scaffold (2C: ) is preferred over substitution at C5 (2D: ) or C6 (2E: ) of ring A for adenosine A1 receptor activity and selectivity in the micromolar range. Furthermore, substitution at the meta (3') position of ring B with chlorine lead to the highly potent and selective adenosine A2A receptor antagonist, compound 2 H: . Compound 2C: and the 2Q: behaved as adenosine A1 receptor antagonists in the performed GTP shift assays. In view of these findings, compounds 2C: , 2 H: , 2Q: and 2P: are potent and selective adenosine A1 and A2A receptor antagonists for the potential treatment of neurological conditions.

Structural Basis for Binding of Allosteric Drug Leads in the Adenosine A1 Receptor.

Despite intense interest in designing positive allosteric modulators (PAMs) as selective drugs of the adenosine A1 receptor (A1AR), structural binding modes of the receptor PAMs remain unknown. Using the first X-ray structure of the A1AR, we have performed all-atom simulations using a robust Gaussian accelerated molecular dynamics (GaMD) technique to determine binding modes of the A1AR allosteric drug leads. Two prototypical PAMs, PD81723 and VCP171, were selected. Each PAM was initially placed at least 20 Å away from the receptor. Extensive GaMD simulations using the AMBER and NAMD simulation packages at different acceleration levels captured spontaneous binding of PAMs to the A1AR. The simulations allowed us to identify low-energy binding modes of the PAMs at an allosteric site formed by the receptor extracellular loop 2 (ECL2), which are highly consistent with mutagenesis experimental data. Furthermore, the PAMs stabilized agonist binding in the receptor. In the absence of PAMs at the ECL2 allosteric site, the agonist sampled a significantly larger conformational space and even dissociated from the A1AR alone. In summary, the GaMD simulations elucidated structural binding modes of the PAMs and provided important insights into allostery in the A1AR, which will greatly facilitate the receptor structure-based drug design.

FSCPX, a Chemical Widely Used as an Irreversible A1 Adenosine Receptor Antagonist, Modifies the Effect of NBTI, a Nucleoside Transport Inhibitor, by Reducing the Interstitial Adenosine Level in the Guinea Pig Atrium.

Based on in silico results, recently we have assumed that FSCPX, an irreversible A1 adenosine receptor antagonist, inhibits the action of NBTI that is apparent on E/c curves of adenosine receptor agonists. As a mechanism for this unexpected effect, we hypothesized that FSCPX might modify the equilibrative and NBTI-sensitive nucleoside transporter (ENT1) in a way that allows ENT1 to transport adenosine but impedes NBTI to inhibit this transport. This assumption implies that our method developed to estimate receptor reserve for agonists with short half-life such as adenosine, in its original form, overestimates the receptor reserve. In this study, therefore, our goals were to experimentally test our assumption on this effect of FSCPX, to improve our receptor reserve-estimating method and then to compare the original and improved forms of this method. Thus, we improved our method and assessed the receptor reserve for the direct negative inotropic effect of adenosine with both forms of this method in guinea pig atria. We have found that FSCPX inhibits the effects of NBTI that are mediated by increasing the interstitial concentration of adenosine of endogenous (but not exogenous) origin. As a mechanism for this action of FSCPX, inhibition of enzymes participating in the interstitial adenosine production can be hypothesized, while modification of ENT1 can be excluded. Furthermore, we have shown that, in comparison with the improved form, the original version of our method overestimates receptor reserve but only to a small extent. Nevertheless, use of the improved form is recommended in the future.

Identification of novel thiazolo[5,4-d]pyrimidine derivatives as human A1 and A2A adenosine receptor antagonists/inverse agonists.

In this study a new set of thiazolo[5,4-d]pyrimidine derivatives was synthesized. These derivatives bear different substituents at positions 2 and 5 of the thiazolopyrimidine core while maintaining a free amino group at position-7. The new compounds were tested for their affinity and potency at human (h) A1, A2A, A2B and A3 adenosine receptors expressed in CHO cells. The results reveal that the higher affinity of these new set of thiazolopyrimidines is toward the hA1 and hA2A adenosine receptors subtypes and is tuned by the substitution pattern at both the 2 and 5 positions of the thiazolopyrimidine nucleus. Functional studies evidenced that the compounds behaved as dual A1/A2A antagonists/inverse agonists. Compound 3, bearing a 5-((2-methoxyphenyl) methylamino) group and a phenyl moiety at position 2, displayed the highest affinity (hA1 Ki = 10.2 nM; hA2A Ki = 4.72 nM) and behaved as a potent A1/A2A antagonist/inverse agonist (hA1 IC50 = 13.4 nM; hA2A IC50 = 5.34 nM).

New potent and selective A1 adenosine receptor antagonists as potential tools for the treatment of gastrointestinal diseases.

The synthesis of 9-alkyl substituted adenine derivatives presenting aromatic groups and cycloalkyl rings in 8- and N6-position, respectively, is reported. The compounds were tested with radioligand binding studies showing, in some cases, a low nanomolar A1 adenosine receptor affinity and a very good selectivity versus the other adenosine receptor subtypes. Functional assays at human adenosine receptors and at a mouse ileum tissue preparation clearly demonstrate the antagonist profile of these molecules, with inhibitory potency at nanomolar level. A molecular modeling study, consisting in docking analysis at the recently reported A1 adenosine receptor crystal structure, was performed for the interpretation of the obtained pharmacological results. The N6-cyclopentyl-9-methyl-8-phenyladenine (17), resulting the most active derivative of the series (Ki = 2.8 nM and IC50 = 14 nM), was also very efficacious in counteracting the effect of the agonist CCPA on mouse ileum contractility. This new compound represents a tool for the development of new agents for the treatment of intestinal diseases as constipation and postoperative ileus.

Evaluation of 2-benzylidene-1-tetralone derivatives as antagonists of A1 and A2A adenosine receptors.

Antagonists of the adenosine receptors (A1 and A2A ) are thought to be beneficial in neurological disorders, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore 2-benzylidene-1-tetralone derivatives as antagonists of A1 and/or A2A adenosine receptors. In general, the test compounds were found to be selective for the A1 adenosine receptor, with only three test compounds possessing affinity for both the A1 and A2A adenosine receptor. The 2-benzylidene-1-tetralones bearing a hydroxyl substituent at either position C5, C6 or C7 of ring A displayed favourable adenosine A1 receptor binding, while C5 hydroxy substitution led to favourable A2A adenosine receptor affinity. Interestingly, para-hydroxy substitution on ring B in combination with ring A bearing a hydroxy at position C6 or C7 provided the 2-benzylidene-1-tetralones with both A1 and A2A adenosine receptor affinity. Compounds 4 and 8 displayed the highest A1 and A2A adenosine receptor affinity with values below 7 µm. Both these compounds behaved as A1 adenosine receptor antagonists in the performed GTP shift assays. In conclusion, the 2-benzylidene-1-tetralone derivatives can be considered as lead compounds to design a new class of dual acting adenosine A1 /A2A receptor antagonists that may have potential in treating both dementia and locomotor deficits in Parkinson's disease.

5-Substituted 2-benzylidene-1-tetralone analogues as A1 and/or A2A antagonists for the potential treatment of neurological conditions.

Adenosine A1 and A2A receptors are attracting great interest as drug targets for their role in cognitive and motor deficits, respectively. Antagonism of both these adenosine receptors may offer therapeutic benefits in complex neurological diseases, such as Alzheimer's and Parkinson's disease. The aim of this study was to explore the affinity and selectivity of 2-benzylidene-1-tetralone derivatives as adenosine A1 and A2A receptor antagonists. Several 5-hydroxy substituted 2-benzylidene-1-tetralone analogues with substituents on ring B were synthesized and assessed as antagonists of the adenosine A1 and A2A receptors via radioligand binding assays. The results indicated that hydroxy substitution in the meta and para position of phenyl ring B, displayed the highest selectivity and affinity for the adenosine A1 receptor with Ki values in the low micromolar range. Replacement of ring B with a 2-amino-pyrimidine moiety led to compound 12 with an increase of affinity and selectivity for the adenosine A2A receptor. These substitution patterns led to enhanced adenosine A1 and A2A receptor binding affinity. The para-substituted 5-hydroxy analogue 3 behaved as an adenosine A1 receptor antagonists in a GTP shift assay performed with rat whole brain membranes expressing adenosine A1 receptors. In conclusion, compounds 3 and 12, showed the best adenosine A1 and A2A receptor affinity respectively, and therefore represent novel adenosine receptor antagonists that may have potential with further structural modifications as drug candidates for neurological disorders.

Imidazo[1,2-α]pyridines possess adenosine A1 receptor affinity for the potential treatment of cognition in neurological disorders.

Previous research has shown that bicyclic 6:5-fused heteroaromatic compounds with two N-atoms have variable degrees of adenosine A1 receptor antagonistic activity. Prompted by this imidazo[1,2-α]pyridine analogues were synthesized and evaluated for their adenosine A1 and A2A receptor affinity via radioligand binding studies and subjected to a GTP shift assay to determine its adenosine A1 receptor agonistic or antagonistic functionality. Imidazo[1,2-α]pyridine, the parent scaffold, was found devoid of affinity for the adenosine A1 and A2A receptors. The influence of substitution on position C2 showed no improvement for either adenosine A1 or A2A receptor affinity. The addition of an amino or a cyclohexylamino group to position C3 also showed no improvement of adenosine A1 or A2A receptor affinity. Surprisingly para-substitution on the phenyl ring at position C2 in combination with a cyclohexylamino group at position C3 led to adenosine A1 receptor affinity in the low micromolar range with compound 4d showing: (1) the highest affinity for the adenosine A1 receptor with a Ki value of 2.06µM and (2) adenosine A1 receptor antagonistic properties. This pilot study concludes that para-substituted 3-cyclohexylamino-2-phenyl-imidazo[1,2-α]pyridine analogues represent an interesting scaffold to investigate further structure-activity relationships in the design of novel imidazo[1,2-α]pyridine-based adenosine A1 receptor antagonists for the treatment of neurodegenerative disorders.

Synthesis and Pharmacological Evaluation of Identified and Putative Metabolites of the A1 Adenosine Receptor Antagonist 8-Cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX).

The A1 adenosine receptor (A1 AR) antagonist [18 F]8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine ([18 F]CPFPX), used in imaging human brain A1 ARs by positron emission tomography (PET), is stable in the brain, but rapidly undergoes transformation into one major (3-(3-fluoropropyl)-8-(3-oxocyclopenten-1-yl)-1-propylxanthine, M1) and several minor metabolites in blood. This report describes the synthesis of putative metabolites of CPFPX as standards for the identification of those metabolites. Analysis by (radio)HPLC revealed that extracts of human liver microsomes incubated with no-carrier-added (n.c.a.)[18 F]CPFPX contain the major metabolite, M1, as well as radioactive metabolites corresponding to derivatives functionalized at the cyclopentyl moiety, but no N1-despropyl species or metabolites resulting from functionalization of the N3-fluoropropyl chain. The putative metabolites were found to displace the binding of [3 H]CPFPX to the A1 AR in pig brain cortex at Ki values between 1.9 and 380 nm and the binding of [3 H]ZM241385 to the A2A AR in pig striatum at Ki values >180 nm. One metabolite, a derivative functionalized at the ω-position of the N1-propyl chain, showed high affinity (Ki 2 nm) to and very good selectivity (>9000) for the A1 AR.

Design and synthesis of novel, potent and selective hypoxanthine analogs as adenosine A1 receptor antagonists and their biological evaluation.

Multipronged approach was used to synthesize a library of diverse C-8 cyclopentyl hypoxanthine analogs from a common intermediate III. Several potent and selective compounds were identified and evaluated for pharmacokinetic (PK) properties in Wistar rats. One of the compounds 14 with acceptable PK parameters was selected for testing in in vivo primary acute diuresis model. The compound demonstrated significant diuretic activity in this model.

Structure of the Adenosine A1 Receptor Reveals the Basis for Subtype Selectivity.

The adenosine A1 receptor (A1-AR) is a G-protein-coupled receptor that plays a vital role in cardiac, renal, and neuronal processes but remains poorly targeted by current drugs. We determined a 3.2 Å crystal structure of the A1-AR bound to the selective covalent antagonist, DU172, and identified striking differences to the previously solved adenosine A2A receptor (A2A-AR) structure. Mutational and computational analysis of A1-AR revealed a distinct conformation of the second extracellular loop and a wider extracellular cavity with a secondary binding pocket that can accommodate orthosteric and allosteric ligands. We propose that conformational differences in these regions, rather than amino-acid divergence, underlie drug selectivity between these adenosine receptor subtypes. Our findings provide a molecular basis for AR subtype selectivity with implications for understanding the mechanisms governing allosteric modulation of these receptors, allowing the design of more selective agents for the treatment of ischemia-reperfusion injury, renal pathologies, and neuropathic pain.

The role of 5-arylalkylamino- and 5-piperazino- moieties on the 7-aminopyrazolo[4,3-d]pyrimidine core in affecting adenosine A1 and A2A receptor affinity and selectivity profiles.

New 7-amino-2-phenylpyrazolo[4,3-d]pyrimidine derivatives, substituted at the 5-position with aryl(alkyl)amino- and 4-substituted-piperazin-1-yl- moieties, were synthesized with the aim of targeting human (h) adenosine A1 and/or A2A receptor subtypes. On the whole, the novel derivatives 1-24 shared scarce or no affinities for the off-target hA2B and hA3 ARs. The 5-(4-hydroxyphenethylamino)- derivative 12 showed both good affinity (Ki = 150 nM) and the best selectivity for the hA2A AR while the 5-benzylamino-substituted 5 displayed the best combined hA2A (Ki = 123 nM) and A1 AR affinity (Ki = 25 nM). The 5-phenethylamino moiety (compound 6) achieved nanomolar affinity (Ki = 11 nM) and good selectivity for the hA1 AR. The 5-(N4-substituted-piperazin-1-yl) derivatives 15-24 bind the hA1 AR subtype with affinities falling in the high nanomolar range. A structure-based molecular modeling study was conducted to rationalize the experimental binding data from a molecular point of view using both molecular docking studies and Interaction Energy Fingerprints (IEFs) analysis.[Formula: see text].

Synthesis and evaluation of N-substituted 2-amino-4,5-diarylpyrimidines as selective adenosine A1 receptor antagonists.

We report the synthesis and biological evaluation of new 2-amino-4,5-diarylpyrimidines as selective antagonists at the adenosine A1 receptor. The scaffold they are based upon is a deaza variation of a previously reported collection of 3-amino-5,6-diaryl-1,2,4-triazines, members of which had a subnanomolar affinity but limited selectivity over the A2A subtype. Initially, similar structure-affinity relationships at the 5-aryl ring were established, and then emphasis was put on increasing selectivity at the hA1AR by introducing substituents on the N2-position, all the while maintaining a nanomolar affinity. Compound 3z, bearing a trans 4-hydroxycyclohexyl substituent, was identified as a potent (Ki(hA1AR) = 7.7 nM) and selective (Ki(hA2AAR) = 1389 nM) antagonist at the human adenosine A1 receptor. Computational docking was effected at the A1 and A2A subtypes, rationalizing the effect of the 4-hydroxycyclohexyl substituent on selectivity, in relation with the nature of the substituent on the 5-position of the pyrimidine.