Exploring the 2- and 5-positions of the pyrazolo[4,3-d]pyrimidin-7-amino scaffold to target human A1 and A2A adenosine receptors

A new series of 7-aminopyrazolo[4,3-d]pyrimidine derivatives (1–31) were synthesized to evaluate some structural modifications at the 2- and 5-positions aimed at shifting affinity towards the human (h) A_2A adenosine receptor (AR) or both hA_2A and hA₁ ARs. The most active compounds were those featured by a 2-furyl or 5-methylfuran-2-yl moiety at position 5, combined with a benzyl or a substituted-benzyl group at position 2. Several of these derivatives (22–31) displayed nanomolar affinity for the hA_2A AR (K_i = 3.62–57 nM) and slightly lower for the hA₁ ARs, thus showing different degrees (3–22 fold) of hA_2A versus hA₁ selectivity. In particular, the 2-(2-methoxybenzyl)-5-(5-methylfuran-2-yl) derivative 25 possessed the highest hA_2A and hA₁ AR affinities (K_i = 3.62 nM and 18 nM, respectively) and behaved as potent antagonist at both these receptors (cAMP assays). Its 2-(2-hydroxybenzyl) analog 26 also showed a high affinity for the hA_2A AR (K_i = 5.26 nM) and was 22-fold selective versus the hA₁ subtype. Molecular docking investigations performed at the hA_2A AR crystal structure and at a homology model of the hA₁ AR allowed us to represent the hypothetical binding mode of our derivatives and to rationalize the observed SARs.     

1,3-Dialkyl-8-N-substituted benzyloxycarbonylamino-9-deazaxanthines as potent adenosine receptor ligands: Design,synthesis, structure–affinity and structure–selectivity relationships

A number of 1,3-dialkyl-9-deazaxanthines (9-dAXs), bearing a variety of N-substituted benzyloxycarbonylamino substituents at position 8, were prepared and evaluated for their binding affinity to the recombinant human adenosine receptors (hARs), chiefly to the hA_2B and hA_2A AR subtypes. Several ligands endowed with excellent binding affinity to the hA_2B receptors, but low selectivity versus hA_2A and hA₁ were identified. Among these, 1,3-dimethyl-N-3′-thienyl carbamate 15 resulted as the most potent ligand at hA_2B (K_i = 0.8 nM), with a low selectivity versus hA_2A (hA_2A/hA_2B = 12.6) and hA₁ (hA₁/hA_2B = 12.5) and a higher selectivity versus hA₃ (hA₃/hA_2B = 454). When tested in functional assays in vitro, compound 15 exhibited high antagonist activities and efficacies versus both the A_2A and A_2B receptor subtypes, with pA₂ values close to the corresponding pK_is. A comparative analysis of structure–affinity and structure–selectivity relationships of the similar analogues 8-N-substituted benzyloxycarbonylamino- and 8-N-substituted phenoxyacetamido-9-dAXs suggested that their binding modes at the hA_2B and hA_2A ARs may strongly differ. Computational studies help to clarify this striking difference arising from a simple, albeit crucial, structural change, from CH₂OCON to OCH₂CON, in the para-position of the 8-phenyl ring.     

Synthesis and structure–activity relationships of new carbonyl guanidine derivatives as novel dual 5-HT2B and 5-HT7 receptor antagonists

To identify potent dual 5-HT_2B and 5-HT₇ receptor antagonists, we synthesized a series of novel carbonyl guanidine derivatives and examined their structure–activity relationships. Among these compounds, N-(9-hydroxy-9H-fluorene-2-carbonyl)guanidine (10) had a good in vitro profile, that is, potent affinity for human 5-HT_2B and 5-HT₇ receptor subtypes (K_i = 1.8 nM and K_i = 17.6 nM, respectively) and high selectivity over 5-HT_2A, 5-HT_2C, α₁, D₂ and M₁ receptors. Compound 10 also showed a suppressing effect on 5-HT-induced dural protein extravasation in guinea pigs when orally administered.     

Synthesis and pharmacological evaluation of optically pure,novel carbonyl guanidine derivatives as dual 5-HT2B and 5-HT7 receptor antagonists

A series of 9-disubstituted N-(9H-fluorene-2-carbonyl)guanidine derivatives have been discovered as potent and orally active dual 5-HT_2B and 5-HT₇ receptor antagonists. Upon screening several compounds, N-(diaminomethylene)-4′,5′-dihydro-3′H-spiro[fluorene-9,2′-furan]-2-carboxamide (17) exhibited potent affinity for both 5-HT_2B (K_i = 5.1 nM) and 5-HT₇ (K_i = 1.7 nM) receptors with high selectivity over 5-HT_2A, 5-HT_2C, α₁, D₂ and M₁ receptors. Optical resolution of the intermediate carboxylic acid 16 via the formation of diastereomeric salts using chiral alkaloids gave the optically pure compounds (R)-17 and (S)-17. Both enantiomers suppressed 5-HT-induced dural protein extravasation in guinea pigs in a dose-dependent manner and the amount of leaked protein was suppressed to near normal levels when orally administrated at 10 mg/kg. (R)-17 and (S)-17 were therefore selected as candidates for human clinical trials.     

Synthesis and biological evaluation of a series of benzoxazole/benzothiazole-containing 2,3-dihydrobenzo[b][1,4]dioxine derivatives as potential antidepressants

A series of benzoxazole/benzothiazole-2,3-dihydrobenzo[b][1,4]dioxine derivatives (5a–5d and 8a–8j) was synthesized. Compounds were evaluated for binding affinities at the 5-HT_1A and 5-HT_2A receptors. Antidepressant activities of the compounds were screened using the forced swimming test (FST) and the tail suspension test (TST). The results indicated that the compounds exhibited high affinities for the 5-HT_1A and 5-HT_2A receptors and showed a marked antidepressant-like activity. Compound 8g exhibited high affinities for the 5-HT_1A (K_i = 17 nM) and 5-HT_2A (K_i = 0.71 nM) receptors; it also produced a decrease of the immobility time and exhibited potent antidepressant-like effects in the FST and TST in mice.     

Discovery of novel α1-adrenoceptor ligands based on the antipsychotic sertindole suitable for labeling as PET ligands

The synthesis and in vitro preclinical profile of a series of 5-heteroaryl substituted analogs of the antipsychotic drug sertindole are presented. Compounds 1-(4-fluorophenyl)-3-(1-methylpiperidin-4-yl)-5-(pyrimidin-5-yl)-1H-indole (Lu AA27122, 3i) and 1-(4-fluorophenyl)-5-(1-methyl-1H-1,2,4-triazol-3-yl)-3-(1-methylpiperidin-4-yl)-1H-indole (3l) were identified as high affinity α_1A-adrenoceptor ligands with K_i values of 0.52 and 0.16 nM, respectively, and with a >100-fold selectivity versus dopamine D₂ receptors. Compound 3i showed almost equal affinity for α_1B- (K_i = 1.9 nM) and α_1D-adrenoceptors (K_i = 2.5 nM) as for α_1A, as well as moderate affinity for 5-HT_1B (K_i = 13 nM) and 5-HT₆ (K_i = 16 nM) receptors, whereas 3l showed >40-fold selectivity toward all other targets tested. Based on in vitro assays for assessment of permeability rates and extent, it is predicted that both compounds enter the brain of rats, non-human primates, as well as humans, and as such are good candidates to be carried forward for further evaluation as positron emission tomography (PET) ligands.     

Synthesis and biological investigation of new equatorial (β) stereoisomers of 3-aminotropane arylamides with atypical antipsychotic profile

A series of novel 3β-aminotropane derivatives containing a 2-naphthalene or a 2-quinoline moiety was synthesised and evaluated for their affinity for 5-HT_1A, 5-HT_2A and D₂ receptors. Their affinity for the receptors was in the nanomolar to micromolar range. p-Substitution (6c, 6f, 6i, 6l, 6o), as well as substitution with chlorine atoms (6g, 6h, 6i), led to a significant increase in binding affinity for D₂ receptors with compounds 6f (K_i = 0.6 nM), 6c and 6i (K_i = 0.4 nM), having the highest binding affinities. m-Substituted derivatives were the most promising ligands in terms of 5-HT_2A receptor binding affinity whereas 2-quinoline derivatives (10a, 10b) displayed the highest affinity for 5-HT_1AR and were the most selective ligands with K_i = 62.7 nM and K_i = 30.5 nM, respectively. Finally, the selected ligands 6b, 6d, 6e, 6g, 6h, 6k, 6n and 6o, with triple binding activity for the D₂, 5-HT_1A and 5-HT_2A receptors, were subjected to in vivo tests, such as those for induced hypothermia, climbing behaviour and the head twitch response, in order to determine their pharmacological profile. The tested ligands presented neither agonist nor antagonist properties for the 5-HT_1A receptors in the induced hypothermia and lower lip retraction (LLR) tests. All tested compounds displayed antagonistic activity against 5-HT_2A, with 6n and 6o being the most active. Four (6b, 6k, 6n and 6o) out of eight tested compounds could be classified as D₂ antagonists. Additionally, evaluation of metabolic stability was performed for selected ligands, and introduction of halogen atoms into the benzene ring of 6h, 6k, 6n and 6o improved their metabolic stability. The project resulted in the selection of the lead compounds 6n and 6o, which had antipsychotic profiles, combining dopamine D₂-receptor and 5-HT_2A antagonism and metabolic stability.     

Pyrrolidine analogs of GZ-793A: Synthesis and evaluation as inhibitors of the vesicular monoamine transporter-2 (VMAT2)

Central heterocyclic ring size reduction from piperidinyl to pyrrolidinyl in the vesicular monoamine transporter-2 (VMAT2) inhibitor GZ-793A and its analogs resulted in novel N-propane-1,2(R)-diol analogs 11a–i. These compounds were evaluated for their affinity for the dihydrotetrabenazine (DTBZ) binding site on VMAT2 and for their ability to inhibit vesicular dopamine (DA) uptake. The 4-difluoromethoxyphenethyl analog 11f was the most potent inhibitor of [³H]-DTBZ binding (K_i = 560 nM), with 15-fold greater affinity for this site than GZ-793A (K_i = 8.29 μM). Analog 11f also showed similar potency of inhibition of [³H]-DA uptake into vesicles (K_i = 45 nM) compared to that for GZ-793A (K_i = 29 nM). Thus, 11f represents a new water-soluble inhibitor of VMAT function.     

Asymmetric total synthesis and identification of tetrahydroprotoberberine derivatives as new antipsychotic agents possessing a dopamine D1, D2 and serotonin 5-HT1A multi-action profile

An effective and rapid method for the microwave-assisted preparation of the key intermediate for the total synthesis of tetrahydroprotoberberines (THPBs) including l-stepholidine (l-SPD) was developed. Thirty-one THPB derivatives with diverse substituents on A and D ring were synthesized, and their binding affinity to dopamine D₁, D₂ and serotonin 5-HT_1A and 5-HT_2A receptors were determined. Compounds 18k and 18m were identified as partial agonists at the D₁ receptor with K_i values of 50 and 6.3 nM, while both compounds act as D₂ receptor antagonists (K_i = 305 and 145 nM, respectively) and 5-HT_1A receptor full agonists (K_i = 149 and 908 nM, respectively). These two THPBs compounds exerted antipsychotic actions in animal models. Further electrophysiological studies employing single-unit recording in intact animals demonstrated that 18k-excited dopaminergic (DA) neurons are associated with its 5-HT_1A receptor agonistic activity. These results suggest that these two compounds targeted to multiple neurotransmitter receptors may present novel lead drugs with new pharmacological profiles for the treatment of schizophrenia.     

Synthesis and pharmacological characterization of novel xanthine carboxylate amides as A2A adenosine receptor ligands exhibiting bronchospasmolytic activity

The carboxylate amides of 8-phenyl-1,3-dimethylxanthine described herein represent a new series of selective ligands of the adenosine A_2A receptors exhibiting bronchospasmolytic activity. The effects of location of 8-phenyl substitutions on the adenosine receptor (AR) binding affinities of the newly synthesized xanthines have also been studied. The compounds displayed moderate to potent binding affinities toward various adenosine receptor subtypes when evaluated through radioligand binding studies. However, most of the compounds showed the maximum affinity for the A_2A subtype, some with high selectivity versus all other subtypes. Xanthine carboxylate amide 13b with a diethylaminoethylamino moiety at the para-position of the 8-phenylxanthine scaffold was identified as the most potent A_2A adenosine receptor ligand with K_i = 0.06 μM. Similarly potent and highly A_2A-selective are the isovanillin derivatives 16a and 16d. In addition, the newly synthesized xanthine derivatives showed good in vivo bronchospasmolytic activity when tested in guinea pigs.     

Synthesis and evaluation of N6-substituted apioadenosines as potential adenosine A3 receptor modulators

Adenosine receptors (ARs) trigger signal transduction pathways inside the cell when activated by extracellular adenosine. Selective modulation of the A₃AR subtype may be beneficial in controlling diseases such as colorectal cancer and rheumatoid arthritis. Here, we report the synthesis and evaluation of β-d-apio-d-furano- and α-d-apio-l-furanoadenosines and derivatives thereof. Introduction of a 2-methoxy-5-chlorobenzyl group at N⁶ of β-d-apio-d-furanoadenosine afforded an A₃AR antagonist (10c, K_i = 0.98 μM), while a similar modification of an α-d-apio-l-furanoadenosine gave rise to a partial agonist (11c, K_i = 3.07 μM). The structural basis for this difference was examined by docking to an A₃AR model; the antagonist lacked a crucial interaction with Thr94.     

Structure–activity relationships of truncated adenosine derivatives as highly potent and selective human A3 adenosine receptor antagonists

On the basis of potent and selective binding affinity of truncated 4′-thioadenosine derivatives at the human A₃ adenosine receptor (AR), their bioisosteric 4′-oxo derivatives were designed and synthesized from commercially available 2,3-O-isopropylidene-d-erythrono lactone. The derivatives tested in AR binding assays were substituted at the C2 and N⁶ positions. All synthesized nucleosides exhibited potent and selective binding affinity at the human A₃ AR. They were less potent than the corresponding 4′-thio analogues, but showed still selective to other subtypes. The 2-Cl series generally were better than the 2-H series in view of binding affinity and selectivity. Among compounds tested, compound 5d (X = Cl, R = 3-bromobenzyl) showed the highest binding affinity (K_i = 13.0 ± 6.9 nM) at the hA₃ AR with high selectivity (at least 88-fold) in comparison to other AR subtypes. Like the corresponding truncated 4′-thio series, compound 5d antagonized the action of an agonist to inhibit forskolin-stimulated adenylate cyclase in hA₃ AR-expressing CHO cells. Although the 4′-oxo series were less potent than the 4′-thio series, this class of human A₃ AR antagonists is also regarded as another good template for the design of A₃ AR antagonists and for further drug development.     

8-Substituted 1,3-dimethyltetrahydropyrazino[2,1-f]purinediones: Water-soluble adenosine receptor antagonists and monoamine oxidase B inhibitors

Multitarget approaches, i.e., addressing two or more targets simultaneously with a therapeutic agent, are hypothesized to offer additive therapeutic benefit for the treatment of neurodegenerative diseases. Validated targets for the treatment of Parkinson’s disease are, among others, the A_2A adenosine receptor (AR) and the enzyme monoamine oxidase B (MAO-B). Additional blockade of brain A₁ ARs may also be beneficial. We recently described 8-benzyl-substituted tetrahydropyrazino[2,1-f]purinediones as a new lead structure for the development of such multi-target drugs. We have now designed a new series of tetrahydropyrazino[2,1-f]purinediones to extensively explore their structure–activity-relationships. Several compounds blocked human and rat A₁ and A_2AARs at similar concentrations representing dual A₁/A_2A antagonists with high selectivity versus the other AR subtypes. Among the best dual A₁/A_2AAR antagonists were 8-(3-(4-chlorophenyl)propyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (41, K_i human A₁: 65.5 nM, A_2A: 230 nM; K_i rat A₁: 352 nM, A_2A: 316 nM) and 1,3-dimethyl-8-((2-(thiophen-2-yl)thiazol-4-yl)methyl)-6,7,8,9-tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (57, K_i human A₁: 642 nM, A_2A: 203 nM; K_i rat A₁: 166 nM, A_2A: 121 nM). Compound 57 was found to be well water-soluble (0.7 mg/mL) at a physiological pH value of 7.4. One of the new compounds showed triple-target inhibition: (R)-1,3-dimethyl-8-(2,1,3,4-tetrahydronaphthalen-1-yl)-6,7,8,9-tetrahydropyrazino[2,1-f]purine-2,4(1H,3H)-dione (49) was about equipotent at A₁ and A_2AARs and at MAO-B (K_i human A₁: 393 nM, human A_2A: 595 nM, IC₅₀ human MAO-B: 210 nM) thus allowing future in vivo explorations of the intended multi-target approach.     

Design and synthesis of N6-substituted-4′-thioadenosine-5′-uronamides as potent and selective human A3 adenosine receptor agonists

On the basis of a bioisosteric rationale, 4′-thionucleoside analogues of IB-MECA (N⁶-(3-Iodo-benzyl)-9-(5′-methylaminocarbonyl-β-d-ribofuranosyl)adenine), which is a potent and selective A₃ adenosine receptor (AR) agonist, were synthesized from d-gulonic acid γ-lactone. The 4′-thio analogue (5h) of IB-MECA showed extremely high binding affinity (K_i = 0.25 nM) at the human A₃AR and was more potent than IB-MECA (K_i = 1.4 nM). Bulky substituents at the 5′-uronamide position, such as cyclohexyl and 2-methylbenzyl, in this series of 2-H nucleoside derivatives were tolerated in A₃AR binding, although small alkyl analogues were more potent.     

Development of novel adenosine receptor ligands based on the 3-amidocoumarin scaffold

With the aim of finding new adenosine receptor (AR) ligands presenting the 3-amidocoumarin scaffold, a study focusing on the discovery of new chemical entities was carried out. The synthesized compounds 1–8 were evaluated in radioligand binding (A₁, A_2A and A₃) and adenylyl cyclase activity (A_2B) assays in order to determine their affinity for human AR subtypes. The 3-benzamide derivative 4 showed the highest affinity of the whole series and was more than 30-fold selective for the A₃ AR (K_i = 3.24 μM). The current study supported that small structural changes in this scaffold allowed modulating the affinity resulting in novel promising classes of A₁, A_2A, and/or A₃ AR ligands. We also performed docking calculations in hA_2A and hA₃ to identify the hypothetical binding mode for the most active compounds. In addition, some ADME properties were calculated in order to better understand the potential of these compounds as drug candidates.     

Synthesis and structure–activity relationships of new carbonyl guanidine derivatives as novel dual 5-HT2B and 5-HT7 receptor antagonists. Part 2

We previously reported that the novel dual 5-HT_2B and 5-HT₇ receptor antagonist N-(9-hydroxy-9H-fluorene-2-carbonyl)guanidine (4) exerted a suppressing effect on 5-HT-induced dural protein extravasation in guinea pigs. To develop a synthetic strategy, we performed docking studies of lead compound 4 bound to 5-HT_2B and 5-HT₇ receptors, and observed that the carbonyl guanidine group forms a tight interaction network with an active center Asp (D135:5-HT_2B, D162:5-HT₇), Tyr (Y370:5-HT_2B, Y374:5-HT₇) and aromatic residue (W131:5-HT_2B, F158:5-HT₇). Based on molecular modeling results, we optimized the substituents at the 5- to 8-position and 9-position of the fluorene ring and identified N-(diaminomethylene)-9-hydroxy-9-methyl-9H-fluorene-2-carboxamide (24a) exhibits potent affinity for 5-HT_2B (K_i = 4.3 nM) and 5-HT₇ receptor (K_i = 4.3 nM) with high selectivity over 5-HT_2A, 5-HT_2C, α₁, D₂ and M₁ receptors. Compound 24a reversed the hypothermic effect of 5-carboxamidotryptamine (5-CT) in mice and also showed a suppressing effect on 5-HT-induced dural protein extravasation in guinea pigs when orally administered at 30 mg/kg. Compound 24a is therefore a promising candidate for a novel class of anti-migraine agent without any adverse effects.     

Discovery of simplified N2-substituted pyrazolo[3,4-d]pyrimidine derivatives as novel adenosine receptor antagonists: Efficient synthetic approaches,biological evaluations and molecular docking studies

In the present study, a molecular simplification approach was employed to design novel bicyclic pyrazolo[3,4-d]pyrimidine (PP) derivatives from tricyclic pyrazolo[4,3-e]-1,2,4-triazolo-[1,5-c]pyrimidines (PTP) as promising human A₃ adenosine receptor (hA₃AR) antagonists. All the target compounds were synthesized using novel and efficient synthetic schemes and the structure–activity relationship studies of these PPs were explored through the synthesis of a series of PTP analogues with various substituents. Substituents with different lipophilicity and steric hindrance (e.g., alkyl and aryl–alkyl) functions were introduced at N² position of the pyrazole ring, while acyl groups with different electronic properties were introduced at C⁶ position of the bicyclic nucleus to probe both electronic and positional effects. Most of the synthesized derivatives of the PP series presented good affinity at the hA₃AR, as indicated by the low micromolar range of K_i values and among them, compound 63 with N² neopentyl substituents showed most potent hA₃AR affinity with K_i value of 0.9 μM and high selectivity (hA₁AR/hA₃AR = >111 & hA_2AAR/hA₃AR = >111) towards other adenosine receptor subtypes. Interestingly, small isopropyl groups at N² position displayed high affinity at another receptor subtype (hA_2AAR, e.g., compound 55, with K_i hA_2AAR = 0.8 μM), while they were less favorable at the hA₃AR. Molecular docking analysis was also performed to predict the possible binding mode of target compounds inside the hA₃AR and hA_2AAR. Overall, PP derivatives represent promising starting points for new AR antagonists.     

1,3-Dialkyl-substituted tetrahydropyrimido[1,2-f]purine-2,4-diones as multiple target drugs for the potential treatment of neurodegenerative diseases

Adenosine receptors and monoamine oxidases are drug targets for neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease. In the present study we prepared a library of 55 mostly novel tetrahydropyrimido[2,1-f]purinediones with various substituents in the 1- and 3-position (1,3-dimethyl, 1,3-diethyl, 1,3-dipropyl, 1-methyl-3-propargyl) and broad variation in the 9-position. A synthetic strategy to obtain 3-propargyl-substituted tetrahydropyrimido[2,1-f]purinedione derivatives was developed. The new compounds were evaluated for their interaction with all four adenosine receptor subtypes and for their ability to inhibit monoamine oxidases (MAO). Introduction of mono- or di-chloro-substituted phenyl, benzyl or phenethyl residues at N9 of the 1,3-dimethyl series led to the discovery of a novel class of potent MAO-B inhibitors, the most potent compound being 9-(3,4-dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)-dione (21g, IC₅₀ human MAO-B: 0.0629 μM), which displayed high selectivity versus the other investigated targets. Potent dually active A₁/A_2A adenosine receptor antagonists were identified, for example, 9-benzyl-1-methyl-3-propargyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)dione (19f, K_i, human receptors, A₁: 0.249 μM, A_2A: 0.253 μM). Several compounds showed triple-target inhibition, the best compound being 9-(2-methoxybenzyl)-1-methyl-3-(prop-2-ynyl)-6,7,8,9-tetrahydro pyrimido [1,2-f]purine-2,4(1H,3H)-dione (19g, K_i A₁: 0.605 μM, K_i A_2A: 0.417 μM, IC₅₀ MAO-B: 1.80 μM). Compounds inhibiting several different targets involved in neurodegeneration may exhibit additive or even synergistic effects in vivo.     

Synthesis and SAR studies of trisubstituted purinones as potent and selective adenosine A2A receptor antagonists

A series of trisubstituted purinones was synthesized and evaluated as A_2A receptor antagonists. The A_2A structure–activity relationships at the three substituted positions were studied and selectivity against the A₁ receptor was investigated. One antagonist 12o exhibits a K_i of 9 nM in an A_2A binding assay, a K_b of 18 nM in an A_2A cAMP functional assay, and is 220-fold selective over the A₁ receptor.     

NAD-based inhibitors with anticancer potential

Three classes of novel inhibitors of inosine monophosphate dehydrogenase have been prepared and their anti-proliferative properties were evaluated against several cancer cell lines.(1) Mycophenolic adenine dinucleotide analogues (8–13) containing a substituent at the C2 of adenine ring were found to be potent inhibitors of IMPDH (K_i’s in range of 0.6–82 nM) and sub-μM inhibitors of leukemic K562 cell proliferation. (2) Mycophenolic adenosine (d and l) esters (20 and 21) showed a potent inhibition of IMPDH2 (K_i = 102 and K_i = 231 nM, respectively) and inhibition of K562 cell growth (IC₅₀ = 0.5 and IC₅₀ = 1.6 μM). These compounds serve both as inhibitors of the enzyme and as a depot form of mycophenolic acid. The corresponding amide analogue 22, also a potent inhibitor of IMPDH (K_i = 84 nM), did not inhibit cancer cell proliferation. (3) Mycophenolic-(l)- and (d)-valine adenine di-amide derivatives 25 (K_i = 9 nM) and 28 (K_i = 3 nM) were found to be very potent enzymatically, but did not inhibit proliferation of cancer cells.