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. 2013 Jan 14;4(2):254–258. doi: 10.1021/ml3003814

Synthesis of Novel Analogs of Cabergoline: Improving Cardiovascular Safety by Removing 5-HT2B Receptor Agonism

Peter I Dosa †,*, Tim Ward †, Michael A Walters †, Suck Won Kim ‡
PMCID: PMC3629922  NIHMSID: NIHMS436125  PMID: 23606928

Abstract

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The dopamine agonist cabergoline has been used to treat prolactinomas, Parkinson’s disease, Cushing’s disease, and sexual dysfunction. However, its clinical use was severely curtailed when it was found that patients taking cabergoline had an increased risk of developing cardiac-valve regurgitation. This potentially life-threatening condition has been associated with drugs, such as cabergoline, that are 5-HT2B receptor agonists. We prepared analogs of cabergoline and have identified several that have limited or no agonism at the 5-HT2B receptor.

Keywords: Cabergoline, 5-HT2B, ergot alkaloid, sexual dysfunction, dopamine agonist

Cabergoline is a dopamine agonist that has been used clinically to treat prolactinomas,1 Parkinson’s disease,2 and Cushing’s disease3 and has been reported to have positive effects in patients suffering from depression.4,5 We became interested in this ergot alkaloid because of its potential to treat sexual dysfunction. Cabergoline has been used clinically to treat patients suffering from male sexual dysfunction, especially those with elevated prolactin levels.6−8 An elevated level of prolactin is a known risk factor for sexual dysfunction, and cabergoline lowers the level of this hormone by binding to D2 receptors in the pituitary gland and inhibiting prolactin synthesis.9 Cabergoline is also an agonist at the D4 and 5-HT1A receptors,10−12 which are believed to play important roles in regulating sexual function in both males and females.13−16

Unfortunately, cabergoline is also a potent agonist of the 5-HT2B receptor (with a reported Ki of 1.2 nM)11 and, like other 5-HT2B agonists such as nor-dexfenfluramine, is known to cause cardiac-valve regurgitation (CVR) in patients.17,18 This potentially fatal complication has greatly limited the clinical use of cabergoline, especially in indications requiring high doses of the drug. A close analog of cabergoline without the propensity to cause CVR could potentially be of significant therapeutic use.

We believed that it should be possible to find safer analogs of cabergoline using design techniques developed to ameliorate 5-HT2B agonism in other therapeutics. Recent studies have shown that it is indeed possible to find safer analogs of drugs that have seen their clinical use limited due to a tendency to cause CVR. For instance, Arena Pharmaceuticals recently received FDA approval for the antiobesity agent lorcaserin, a 5-HT2C agonist which is structurally related to nor-fenfluramine but with greatly reduced 5-HT2B agonism (Figure 1).19 Clinical studies showed that, unlike nor-dexfenfluramine, this drug does not appear to cause heart-valve damage in patients.20

Figure 1.

Figure 1

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The antiobesity agents nor-dexfenfluramine and lorcaserin are both potent 5-HT2C agonists, but lorcaserin is a much weaker agonist at the 5-HT2B receptor and, unlike nor-dexfenfluramine, does not cause heart-valve damage clinically.

As the connection between 5-HT2B agonists and CVR had not yet been discovered when cabergoline was first developed, no apparent effort was made to minimize 5-HT2B agonism at that time.21 There are two strategies for removing the potential to cause heart-valve damage from cabergoline analogs: either their activity at the 5-HT2B receptor can be reduced, or they can be converted from agonists into antagonists, which are not believed to cause CVR.22,23 We set out to try both approaches.

We began our efforts to reduce 5-HT2B activity by replacing the dimethylamino side chain of cabergoline with a less basic morpholine group (compound 1 in Figure 2), as a similar change has previously been shown to help reduce activity at 5-HT2 receptors.24 The synthesis of 1 proved to be more problematic than expected. Most published syntheses of cabergoline start from cabergolinic acid 2 or its methyl ester 3, shown in Schemes 2 and 1, respectively.21,25 However, extensive attempts to acquire either of these compounds commercially were unsuccessful. Fortunately, it is possible to synthesize 3 directly from cabergoline using the method of Wang and co-workers, who found that simply stirring cabergoline in methanol and triethylamine leads to the near quantitative recovery of methyl ester 3 (Scheme 1).26 Further investigation showed that it was possible to bypass methyl ester 3 and instead generate amide 4 directly in one step by heating cabergoline with 3-morpholinopropylamine (Scheme 1). An attempt to access the desired final N-acylurea 1 directly from amide 4 by heating 4 with ethyl isocyanate21 led to an inseparable mixture of products. We instead used a multistep synthetic route through Boc-protected phenyl carbamate 5 to obtain 1 from 4 (Scheme 1).25 As this multistep synthesis proved to be rather time-consuming, we later developed a faster route to 1, by reacting cabergolinic acid (2) with the morpholino-analog of EDC, which was prepared from 3-morpholinopropylamine and ethyl isocyanate (Scheme 2).

Figure 2.

Figure 2

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Hypothesis 1: Lowering the pKa of the basic amine could decrease 5-HT2B activity.

Scheme 2. Alternate Synthesis of Morpholino Analog 1.

Scheme 2

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Reagents and conditions: (a) i. NaOH, H2O, methanol; ii. HCl. (b) i. CH2Cl2, 0 °C; ii. NEt3, p-toluenesulfonyl chloride, reflux. (c) NEt3, CH2Cl2.

Scheme 1. Synthesis of Morpholino Analog of Cabergoline.

Scheme 1

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Reagents and conditions: (a) Methanol, NEt3. (b) 3-Morpholinopropylamine, 65 °C. (c) i. Boc2O, DMAP, THF; ii. NaHMDS; iii. PhOCOCl. (d) i. EtNH2·HCl, 2-propanol, 50 °C; ii. HCl, H2O.

Compound 1 and cabergoline were evaluated in a competitive binding assay at the 5-HT2B receptor against 125I-radiolabeled 2,5-dimethoxy-4-iodoamphetamine, as well as in a 5-HT2B functional assay using HTRF quantitation of IP1 accumulation. Unfortunately, the results of these assays showed that compound 1 (Ki 7.1 nM in the binding assay, EC50 13 nM in the functional assay) was not significantly less active at the 5-HT2B receptor than cabergoline (Ki 1.4 nM, EC50 13 nM). As expected, both compounds were full agonists at the 5-HT2B receptor (see Figure 4 for a listing of all in vitro data and the structures of compounds evaluated).

Figure 4.

Figure 4

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Structure and activity of cabergoline and analogs in 5-HT2B binding (Ki) and functional assays (EC50 for agonists; IC50 and KB for antagonists).

We then turned our efforts to finding analogs of cabergoline that were 5-HT2B antagonists instead of agonists. We based our initial approach on the results of a study by Pertz and co-workers, who found that while the ergot alkaloid pergolide (which is 6-propyl substituted) is a functional agonist at 5-HT2B receptors in porcine pulmonary arteries, 6-methylpergolide is a 5-HT2B antagonist (Figure 3).27 Both compounds are full agonists at the D2 receptor. We therefore decided to prepare an analog of cabergoline in which the 6-allyl moiety is replaced by a methyl group. As we were also unable to obtain the 6-methyl analog of compound 3 commercially, we instead began our synthesis by hydrolyzing dihydroergotamine and converting the resulting acid into a methyl ester in order to ease purification (Scheme 3). From methyl ester 7, hydrolysis followed by coupling to EDC21 yielded the desired 6-methylated cabergoline analog 8.

Figure 3.

Figure 3

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Replacing the 6-propyl group of pergolide with a methyl group switches the functionality at the 5-HT2B receptor but not at the D2 receptor.

Scheme 3. Synthesis of 6-Methylcabergoline.

Scheme 3

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Reagents and conditions: (a) i. NaOH, H2O, methanol; ii. H2SO4, methanol. (b) NaOH, H2O, methanol. (c) NEt3, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide HCl, CH2Cl2.

Compound 8 demonstrated similar potency as cabergoline and compound 1 in a 5-HT2B radioactive binding assay (Ki 2.2 nM; see Figure 4). A 5-HT2B functional assay showed that the change from a 6-allyl group to a 6-methyl group did significantly reduce 5-HT2B agonism but was not enough to completely eliminate it, as compound 8 is a partial agonist, with an EC50 of 16 nM and a Emax of 38%.

After we finished our synthesis of 6-methylcabergoline, a new publication from the Pertz group reported that 6-methylcabergoline was a functional antagonist at endothelial 5-HT2B receptors in porcine pulmonary arteries.28 Because of the results obtained by Pertz, we retested compound 8 in a different human 5-HT2B functional assay in which intracellular calcium mobilization was monitored using a FLIPR Tetra fluorescence imaging plate reader. Compound 8 was consistently a partial agonist at the human 5-HT2B receptor in this assay, though the Emax varied from 5 to 49% (n = 5, average Emax of 23%). We believe that the most likely explanation for this discrepancy is interspecies differences in the 5-HT2B receptor. While the pig and human 5-HT2B receptors have been reported to possess a rather close 95% sequence homology,29 there have been previous reports of compounds having significantly different activities at receptors having similar levels of interspecies homology, including at least two prior examples with ergot alkaloids at serotonin receptors.30,31

In an attempt to find compounds with less agonism at the human 5-HT2B receptor, we next directed our attention to the preparation of 9 (shown in Scheme 4), the indole-methylated analog of 8. We based this choice of target on the example of the ergot alkaloid methylergonovine, which is a partial agonist at the 5-HT2B receptor, and its indole-methylated analog methysergide, which has significantly weaker activity at the receptor in both binding and functional assays.17 Rather than begin the synthesis of 9 from dihydroergotamine, we chose instead to start from metergoline, which is significantly less expensive (1/25 the cost per mole) and already has a methyl group on the indole in place. Metergoline was first Cbz-deprotected with Pd/C and hydrogen in nearly quantitative yield to furnish 10 (Scheme 4). This primary amine was then converted into an aldehyde using a biomimetic procedure developed by Rapoport.32 First the amine was condensed with 4-formyl-1-methylpyridinium benzenesulfonate to form a Schiff-base intermediate. The resulting imine was then deprotonated with DBU to form an anion (with resonance structures 11a and 11b). Next, protonation with oxalic acid occurred preferentially on the carbon closer to the highly electron-withdrawing quaternized pyridine nitrogen32 and the resulting imine was then hydrolyzed to yield crude aldehyde 12. This material was oxidized with Ag2O33 and converted into a methyl ester with H2SO4 and methanol in an overall 32% yield from metergoline. This procedure also yielded varying amounts of dimethyl acetal 15 (shown in Figure 4), which is presumably formed if not all aldehyde 12 is oxidized by Ag2O prior to H2SO4 and methanol treatment. From methyl ester 13, acetic acid catalyzed amide formation21 furnished 14 in good yield. Finally, heating amide 14 with ethyl isocyanate gave the desired final product 9.

Scheme 4. Synthesis of 1,6-Dimethylcabergoline (9).

Scheme 4

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Reagents and conditions: (a) Pd/C, H2, methanol. (b) CH2Cl2/DMF. (c) DBU. (d) Oxalic acid in H2O, 0 °C to rt. (e) Ag2O, THF, methanol, 10% NaOH in H2O. (f) H2SO4, methanol. (g) 3-(Dimethylamino)-1-propylamine; acetic acid, 120 °C. (h) Ethyl isocyanate, toluene, 115 °C.

Compounds 9 and 15 showed no agonism in the IP1 5-HT2B functional assay (n = 1) or in the intracellular calcium mobilization 5-HT2B functional assay (n = 5), while compound 14 showed minimal to no agonism in these assays (Emax < 12% in all cases). All three compounds were potent antagonists against serotonin at the 5-HT2B receptor. These results demonstrated that it is indeed possible to synthesize a close analog of cabergoline in which 5-HT2B agonism has been eliminated. Further investigation into 9 revealed that it is a full D4 agonist, a potent but partial D2 agonist (EC50 1.4 nM, Emax 50%), and a weak 5-HT2A antagonist (IC50 11.5 μM).

When the screening results for all the products synthesized are taken as a whole, it is possible to make two tentative conclusions about the SAR of these novel cabergoline analogs. First, indole-methylation and replacement of the 6-allyl with 6-methyl both reduce the degree of functional agonism at the 5-HT2B receptor. Second, while several of the compounds that were made had very different substitutions at the 8-position of the ergot alkaloid, the 5-HT2B binding activity varied remarkably little among these compounds. These substitutions also had only a small effect on 5-HT2B functional activity. These results suggest that any future efforts to reduce 5-HT2B binding activity should focus elsewhere on the scaffold.

Our future plan is to test these compounds in animal models of SSRI-induced sexual dysfunction. Sexual dysfunction is a common side effect of SSRI administration and a leading cause of patient noncompliance.34 Dopamine agonists, including cabergoline, have been reported to reverse SSRI-induced sexual dysfunction in some patients,35−37 and we intend to use our cabergoline analogs to help establish the pharmacological profile necessary for a drug to have therapeutic efficacy against this disorder in both men and women.

Acknowledgments

5-HT2B binding and IP1 5-HT2B functional assays were conducted by CEREP (France). All other functional data were generously provided by the National Institute of Mental Health’s Psychoactive Drug Screening Program, Contract # HHSN-271-2008-00025-C (NIMH PDSP). The NIMH PDSP is directed by Bryan L. Roth MD, Ph.D., at the University of North Carolina at Chapel Hill and Project Officer Jamie Driscoll at NIMH, Bethesda MD, USA.

Supporting Information Available

Experimental details of in vitro assays, synthetic procedures, and characterization of all final compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

This research was supported by a Grant Writers Workshop Seed Grant from the College of Pharmacy at the University of Minnesota.

The authors declare no competing financial interest.

Funding Statement

National Institutes of Health, United States

Supplementary Material

ml3003814_si_001.pdf (239.3KB, pdf)

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Supplementary Materials

ml3003814_si_001.pdf (239.3KB, pdf)

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