Natural products: a continuing source of inspiration for the medicinal chemist

Iwu and Wootton (eds.), Ethnomedicine and Drug Discovery 9 2002 Elsevier Science B.V. All rights reserved.

61

CHAPTER 5

Natural products" a continuing source of inspiration for the medicinal chemist

S MBUA NGALE EFANGE

Abstract The study of naturally occurring compounds has led to innumerable advances in pharmacology and medicine. For the medicinal chemist, a biologically active natural compound provides a useful template for designing novel molecules, which can be used to probe the mechanism of action of the parent compound, or to develop drugs with improved pharmacological profiles. The discipline of medicinal chemistry is therefore crucial to modern drug development. Developing countries are richly endowed with flora containing many useful natural products. Unfortunately, little effort has been made in these same countries to develop the expertise (in areas such as medicinal chemistry) required to mount a credible drugdevelopment effort. The lack of expertise in modern drug-development methods has relegated most developing countries to the role of raw material supplier. To address this inadequacy, training programs in modern drug design and development should become an integral part of national policy in the South. In the subsequent presentation, two case studies are provided to illustrate the use of natural products in drug design.

Keywords: pharmacology, medicinal chemistry, drug-development, natural products

I. I n t r o d u c t i o n Modern medicine traces its origins to Man's use of plant and animal parts in alleviating human suffering caused by disease. Some of the earliest compilations of the medicinal uses of plants include the De materia medica of Dioscorides (AD 78) and the Egyptian Papyrus Ebers (1550 BC) (reviewed in Ref. 1). It is estimated that as recently as 1985, over 120 pharmaceutical products in use were derived from plants. 2 Moreover, up to 75% of these agents appear to have been discovered by examining the use of these plants in traditional medicine. According to some estimates, there are at least 250,000 plant species in existence on this planet (most of which are found in the tropical rain forest). Therefore, it is fair to conclude that plants remain an important source of new drugs. The benefits of plant products extend far beyond their immediate use as medications. A review of the scientific literature shows that natural products have contributed immensely to the advancement of pharmacology and physiology. Historically, validation of the medicinal uses of a given plant has been invariably

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Ethnomedicine and drug discovery

followed by attempts to isolate and determine the structure of the active ingredient(s). In the ensuing investigation of the isolated compound, new insights into the mechanism of drug action have been revealed, new areas of pharmacology have been born, and new drugs have been developed. 3 In this connection, the annals of modern pharmacology have recorded impressive gains from natural products such as heroin, nicotine, acetylcholine, penicillin. Since molecular structure is intricately linked to biological activity, for the medicinal chemist, the natural product represents a treasure trove of possibilities. When the mechanism of action of a compound is unknown, the synthesis and study of carefully designed analogs of the lead compound can provide deeper insights into the interactions of the compound with the biological target. If the mode of action of the compound is known, modification of the lead structure can be used to fine-tune the drug-molecular target interaction so as to produce the desired biological response. Finally, lead modification is also employed simply to alter the physicochemical characteristics of a given molecule so that the latter can be amenable to formulation (e.g. as an oral dosage form). In the following examples, we demonstrate (a) how lead modification was used to simplify molecular structure while maintaining the desired pharmacological characteristics, and (b) how knowledge of the mechanism of action of a natural product was used in the design of a subsequent generation of pharmacologically active agents.

II. Example 1" Design of ibogaine analogs Ibogaine (1, Figure 1) is the major constituent of the root ofTabernanthe iboga, a shrub commonly found in West and Central Africa. 4'5 Ibogaine has been shown to reduce cocaine and morphine self-administration in laboratory animals (reviewed in Ref. 6 ), and to attenuate alcohol intake by alcohol-preferring rats. 7 The compound also reduces the rewarding effects of nicotine in animals. 8'9 In addition, the compound reduces craving in humans. 10-13 Ibogaine is thus a potentially useful anti-addictive agent. The goal of this study was to develop synthetic ibogaine-like compounds that might be more effective than the parent compound. Due to the fact that ibogaine is a relatively complex structure, our immediate objective was to identify the simplest ibogaine fragment that retains anti-addictive activity. The fragmentation approach has been used extensively in medicinal chemistry to probe ligand-receptor interactions and to generate novel molecular entities. 14 A close inspection of ibogaine reveals that the latter is composed of two major fragments, 2 and 3 (Figure 1), derived from 5-methoxytryptamine and isoquinuclidine. An intermediate structure, which incorporates elements of both fragments, is the tricyclic heterocycle 4 (Figure 1). In previous studies of conformationally restricted tryptamines, compounds derived from this basic skeleton had been found to antagonize aggressive behavior in animals 15 and to display potential antidepressant activity. 16 The current study investigated the biological activities of the corresponding azepino[4,5-b]bezothiophenes. The target compounds (12a-d) were synthesized as shown in Figure 2. When tested in vitro (Table 1), all four compounds displayed moderately high affinity for

63

Natural products

MeO.,~~

\N

N H lbogaine (1)

MeO H 2

% MeO

N/,~

4 Fig. 1. Ibogaine and selected ibogaine fragments.

dopamine and serotonin transporters, 17 two molecular targets implicated in the mode of action of cocaine 18'19 and ibogaine. 2~ The compounds also interact with other molecular targets with varying degrees of effectiveness and thus appear to mimic the parent compound. In subsequent studies, compounds 12a, 12c and 12d were found to dose-dependently reduce cocaine self-administration in rats without affecting general locomotor activity (Efange et al., unpublished). Taken together, the foregoing investigations confirmed that the tricyclic ibogaine fragment 4 retains antiaddictive activity and is thus a useful platform for developing new anti-addictive agents.

64

Ethnomedicine and drug discovery

/"-CI

5

~

NHEt -.,11

e

N

7'

6

[ ~ S

C

[ ~ S

NHAc --4

10

8

9

~N~CH3 _

OH

~ .q

N~ CH3

_

y

12

R R'

R

a:R-R'=H b:R-Br c: R - NO2, R ' - H

R'

d:R=R'=CI

a" CH20(aq.), HCl(g), conc. HCI"b: KCN, PTC,H20,90-95~ c: LiAIH4,ether,AlCl3, reflux; d" Ac20, Et3N;e: BH3.THF,reflux; f: styreneoxide, EtOH,reflux; g" CF3CO2H,conc. H2SO4(cat.). Fig. 2. Synthesis of substituted hexahydroazepino[4,5-b]benzothiophenes.

Table 1 Relative Affinities (ICs0 • SD, #M) of hexahydroazepino[4,5-b]benzothiophenes and Reference Compounds at Selected Molecular Targets*

Target

DAT (WIN35,428)

SERT (RTI-55)

D1 (SCH23390)

D2 (YM-091512)

D3 (7-OHDPAT)

12a 12b 12c 12d Ibogaine Noribogaine

0.14 0.21 0.25 0.25 4.11 3.35

0.30 4.30 1.10 1.20 0.59 0.04

3 • 0.05 5 • 0.05 5 i 0.06 70 + 0.09 > 10a > 10a

5.0 + 3.5 + 3.1 • 5.0 • > 10a > 10a

0.6 2.0 0.6 2.0

• • + • • •

0.003 0.02 0.01 0.001 0.45 a 0.50 a

• 0.001 • 0.01 • 0.002 • 0.002 • 0.09 a -+- 0.01 a

0.07 0.28 0.14 0.21

*Data taken from ref. 17. Values shown for reference compounds are Ki _+ SD, ~M.

+ + + +

>10 a >10 a

0.01 0.28 0.04 0.14

Natural products

65

III. Example 2: Design of dual modulators of serotonergic and opioidergic function At pharmacologically relevant doses, ibogaine interacts with several molecular targets implicated in drug dependence, including serotonin transporters, 2~ mu and kappa opioid receptors, 22-24 NMDA, 25-29 sigma 3~ and nicotinic acetylcholine receptors. 32'33 These observations have led to the suggestion that the anti-addictive properties of this molecule derive from its simultaneous interaction with multiple molecular targets. To investigate this possibility, we chose to develop compounds that can modulate central nervous system function by interacting with both serotonin reuptake sites and opioid receptors, two molecular targets implicated in psychostimulant dependence. Previously, Hitzig 34 had observed synergistic effects in the treatment of alcohol dependence with a combination of dopamine and serotonin agonists. Therefore, it was expected that dual modulators of serotonergic and opioidergic function would display more useful anti-addiction profiles than agents directed solely at the individual molecular sites. Inspiration for the design of this class of molecules came from an inspection of the structures of ibogaine and morphine, two important natural products. As shown in Figure 3, a tryptaminyl (2, Figure 3) or tryptamine-like (13, Figure 3) fragment can be found embedded in both natural compounds. While ibogaine recognizes both serotonin reuptake sites and opioid receptors, morphine only recognizes opioid sites. For this exercise, it was postulated that a suitably designed tryptamine-like compound could be made to recognize both serotonin reuptake sites and one or more classes of opioid receptors. After further analysis and modeling, the spiro compound 14 (Figure 3) emerged as the consensus structure for this drug discovery effort. Two noteworthy design features are" a) incorporation of the flexible aminoethyl side chain of the tryptaminyl fragment within a five-membered spirofused heterocycle; and b) the introduction of a C3-aryl group. Conformational restriction in part [a] was employed to hold the aminoethyl position in a suitable orientation for opioid receptor binding and to reduce the number of molecular targets that can recognize this structure. Indeed, spiro[indan-l,3'-pyrrolidine] derivatives, synthesized as conformationally restricted profadol analogs, had been shown to alleviate pain in animals, presumably via opioid receptors. 35'36 The C3-aryl group was introduced because 3-phenyl-l-indamines are known to inhibit monoamine reuptake. 37 The presence of the C3 substituent introduces a second chiral center. Therefore, two sets of diastereomers are possible. For the initial effort, the syn diastereomers 21 a-e were synthesized as shown in Figure 4 and tested in vitro (Table 2). 38 The compounds were tested for binding at selected binding sites with the aid of radioligand binding techniques. The spiroindanamines were tested as the hydrochlorides in racemic form. SERT refers to the cocaine binding site on the serotonin transporter. DAT is the dopamine transporter, and mu and kappa refer to the mu and kappa opioid receptors. Affinities are described as high (< 0.1 ~tM); moderate (0.1-1 laM), weak (1-10), and poor (> 10 ~M). Of the four compounds tested, the N-cyclopropylmethyl analog 21e consistently displayed the lowest affinity for monoamine transporters. The other three

.eO

Ethnomedicine and drug discovery

66

H Ibogaine (1)

HO

0~" OH Morphine

L

r-- NCH3

H

2

13

Aryl 14 Fig. 3. Design of 3-arylspiro[indan-l,3t-pyrrolidine] analogs. Table 2 Relative Affinities (ICso, #M) of spiro[indan-l,4t-pyrrolidine] Analogues for Monoamine Transporters and Opioid Receptors* Target

SERT(RTI-55)

DAT([3H]WIN) #-OPIOID([ 125 I]DAMGO

21a 21c 21d 21e Fluoxetine GBR12935 Naloxone U50488

0.01 #M 0.002 0.75 2.41 0.O34

0.2 #M

0.5 #M

10 #M

0.15 0.73 2.9

5.0 2.9 0.98

80 1.52 1.0

~;-OPIOID([3H]U69593)

0.002 0.002 0.003

*Data taken from ref. 38. Ki values of reference compounds are provided for comparison. Relative affinities were determined by previously published methods using the radioligands shown in the table above.

67

Natural products 0

OH

CI

1._.66

17

R' " - ~ " ~

15

ct o

H

~ NC

CN

"~- CN

R

20

1_.99

f or f,g

Series Series Series Series

a b: c: d

1...88

R = R'= H II R = 3,4-CI 2; a'-- H R = 3,4-CI 2; R'= 6-OMe R = 3,4-CI 2; a ' = 6-OH

/',,. /L~

H

....,1 N'

14a

21a-c

h

~"

N'

21e

a: NaBH4, MeOH, r.t. b: SOCI 2, pyridine, 0~ to r.t. c: NaCN, DMF, KI (cat.), 70 ~ d: BrCH2CN, LDA,THF,-78 ~ to-60 ~ to r.t. e: 78% H2SO4-HOAc (w/v), 125 ~ f: BH3.THF, reflux; g: 50% HBr-HOAc, reflux; h: cycIopropylmethyl bromide, K 2003, DMF, r.t.

Fig. 4. Synthesisof 3-arylspiro[indan-l,3'-pyrrolidine]analogs.

compounds (21a, c, d) showed moderate to high affinity for these transporter sites. The compounds also displayed some affinity, albeit moderate, for opioid receptors. In this respect, compound 21 e was the most potent at mu opioid receptors, while 21 e was the most potent ligand at kappa opioid receptors. We conclude, then, that

68

Ethnomedicine and drug discovery

compounds in this class recognize both serotonin transporters and opioid receptors. Additional investigations will reveal the intrinsic activity of these compounds at opioid receptors and their potential utility as pharmacologic agents.

IV. C o n c l u s i o n Tropical forests harbor a vast abundance of plant life and continue to serve as a useful source of new natural products. Unfortunately, the source countries have done little to capitalize on the enormous potential of natural products as lead compounds in a broader drug development effort. The lack of participation in this value-added area of drug development has relegated most developing countries to the role of raw material supplier. For the reason that lead compounds can inspire the development of novel molecular entities, it is safe to say that many developing countries now import pharmaceutical products that were inspired by natural products obtained from the very same countries. The development of expertise and infrastructure that can capitalize on valuable natural resources should, therefore, become a priority at the dawn of the Third Millennium.

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