Journal of Scientific & Indu strial Research Vol. 62, June 2003, pp 537-553

Selective Phosphodiesterase 4 Inhibitors - Emerging Trends in Astluna Therapy (Antiasthmatics-3) Ranju Gupta*, Oeepika Gandhi and Oharam Paul Jindal University In stitute of Pharmaceuti cal Sciences, Panjab Universit y, Chandigarh 160014

Considerable interest has been generated in th e potential utility of isozyme selective inhibitors of phosph odiesterases in the treatment of and other inflali1matory disorders. Heterogeneity in ti ssue distribution as well as their different fun cti onal roles make these enzymes very attractive targets for medicinal chemi sts. To date at least II different fami lies of POE isozymes are known, among which POE 4 plays a major rol e in mod ul ating the activity of virtu ally all cell s involved in the inflammatory process. In hibitors of thi s enzyme family display impressive antiasthmatic ac ti vity by red ucing th e bronchial smooth mu scle tone and consid erable anti-inflammatory activity. The review details th e various classes of POE 4 inhibitors stru cturall y related to , nitraquazone and xa nthines, which appear to be very attractive models for synthesis of novel selecti ve POE 4 inhibitors potentially useful for the treatment of asthma and chronic obstructive pulmonary di seases. Rationale for the use of POE4 inhibitors in th e treatment of asthma is also discussed.

Key words: Phosphodiesterase 4 inhibitors, Asthma therapy, Antiasthmatics, Isozyme, Rol ipram, Nitraquazonc. Xanthincs.

Introduction Phosphodiesterase Superfamily There has been growing interest in recent years At least e leven different fami li es of POE tn the utility of selective phosphodiesterase (POE) isozymes are known based on the ir substrate inhibitors as novel targets for drug d iscovery. specificity (cAMP or cGMP) kinetics, and th e ir Phosphodiesterases compri se a heterogenous group responsi veness to a ll osterics modulator and of isozymes which differ neverthe less in their e ndogenous or exogenous regulators 1-9 (Table I). kinetic and physical characteristics, substrate Each POE fam il y in c ludes subfamilies that are (cAMP or cGMP) selectivities, sensitivity to e ncoded by distinct genes but have 70-80 per cent e nd ogenous activators, susceptibility and response homology with one anoth er and finally contain to phosphorylation by protein kinase, tissue several members. The fami Ii es of POEs are re ferred l distribution and subce llular locali zation -, . These by Arabic nume ral s (1-10), subfamilies by capital isozymes hydrolyse the 3'-phosphodiester bond of letters (A-D) and members by Arabic numeral s (1-3). cyclic messengers cAMP or cGMP to their inactive Nearly all these different POEs have unique primary S'-nucleotide forms. cAMP and cGMP are sequence in the ir catalytic or regulato ry domains and ubiquitous intracellular second messengers which they are often selectively expressed. This implies that playa promin e nt role in the regulation of important it may be possible to modulate individual isozymes using specific drugs. The differing distribution of cellular fu nctio ns such as secretion, contraction, POEs between cell types, the large diversity in metabolism and growth. T he e levation of their structure, function and regulation among POE intrace llular levels by POE inhibitors represents a isozymes makes it possible to develop selective and useful strategy for eliciting a variety of possibl y the rapeutically useful inhibitors and pharmacologica l effects. activators of individual isozymes, which will be devoid of unnecessary side effects. A number of * Author for correspondence inhibitors of phosphodiesterase isozymes have been ranju29in @yahoo.co.in described). They can be used as cardiotonics, 538 J SC I IND RES VOL 62 JU E 2003

Tab i<.: 1- Charactcri sti cs of ph os ph odiesterase isozyme families. mediators, cytoki nes and growth factors. Acute Famil y Isozyme Binding Specific Loca li zation airway obstlUction, bronchial hyperresponsiveness (s ubfalllil y) spec ifi cit y inhibitors ancl inflammatory state of bronchial mucosa with increased level of inflammatory mediators I(A .B.C) Calciulll/ cA MP or . Nerve ti ss ue. cha racteri se this pathology. Eosin ophil s also playa calmodulin cGMP phenothi azine s. stilllulated tracheal prominent role in asthma sugge ting it to be an 8 11 epithelium. inflammatory disorder . . Today asthma is affecting T cells over 5 per cent of th e adu lt populat ion and perhaps 2A( I-:l) cGMP Low affinit y Erythro[ hydrox Plat elets, upto 10 per cent of children. It is rising in prevalence, stilllulated fo r cA MP yl-nonyl] - endothelium, and cGMP adenine Illacrophagcs severity and mortalit y in the devel oping countries :l(A. B) cGMP Hi gh Si guazodan, SlIlooth lIluscle, despite substan ti al in crease in number of newer H inhibited affinit y for cil ostazo l. T cel ls. . antiasthmatic agents . Many clas ses of dru gs like cAMP lIlilrinone. macrophagcs. leu kotriene receptor antagon ists, 5- 1i pox ygenase cil ostamide epithelial cells. adipocytes inhibitors, adenosin e antagonists. 'l11ti cholinergics , ~ 2 -ago ni s t s, glucocorti coid s and an tihi stamines are 4(A.B.CD) cAMP High RO-20- 1724. Inflammatory 15 II spec ifi c affinit y for ro li pralll. cells. neuronal al so used as effecti ve bronchodiiators· . . cA MP CDP-8 .J O. ti ss ue ~ 2 Ad r e n ocepto r agonists are wid ely used in asthma SB -207499, therapy and on inhalation prod uce in stant RP-7J40 1 bronchodilation. They however have littl e effect on SeA) cGMP Hi gh Za prinast. Platelets. lu ng specifi c aflinity for S K&F-962:l 1; epithelia l cells underl ying inflammation and becau - of thi s reason cGMP dipyridilllolc their regul ar and long term use has been debated 8. 12-15 . 6(A.B.CD) Photo- Hi gh for Dipyridilllole. Retinal rod s and Systemic side effects such as tac hycard ia, pa lpitatio n recept or cGMP, Iow za prinast cones and headache are observed with inh aled B­ for cAMP adrenoceptor agoni sts and also the self-admin istration 7(A. B,CD) Rolipmm Hi gh None T cells, B cell s. of inhaled drugs may be difficult for old and disabled insensiti ve aflinity for skeletal mu scle. cAMP hean persons. s on the other hand have we ll documented anti-inflammatory properties and can reduce ai rway hyperresponsiveness bu t lack ant ithrombotic agents, vascul ar and airway smooth l 2 immedi ate bronchodilatory action . Chron ic muscle relaxants, anti-i nfl ammatory agents and administrati on of can cause antidepressants. The knowledge that - (a) multiple significant suppression of hypothalamu s pituitary distin ct POE isozymes ex ist, (b) Isozymes differ in adrenal (HPA) ax is and of bone growth in children. It their cellular di stribution ancl synthesis of a variety of may also affect di sease progression. Two detai led inhibitors possessing marked degree of selectivity for 16 17 reviews have been publi shed by l indal ei al. . on one isozyme over other, raises the possibility th at the Bradrenoceptor agoni sts and methylxant hines as unfavourabl e side effects profil e of nonselecti ve POE antiasthmati cs. in hibitors can be reduced by sy nthesizing compounds that are targeted specifical ly for the isozymes that Novel ora ll y acti ve anti asthmat ic drugs whi ch predominates in the particular tissue or th e cell of di splay both bro nchod il atory and an ti -infl ammatory I. nterest to . profile with reduced side effects and also wh ich can Recently, selective phosphodiesterase inhibitors substitute the combined therapy of inhaled B2- have received considerable attention ' as molecul ar receptor agonists (bronchodi lators) and targets fo r the development of antiasthmat ic agents. cort icosteroid s (anti-inflammatory) is the need of Several comprehensive reviews have been published hour. One class of drugs, whi ch we can look forward recently on the use of POE 4 inhibitors in the for effective as thma therapy with both treatment of asthma3.7.8. 11 .12. bronchodil atory and glucocorticoid like anti- infl ammatory property can be selecti ve Phosphodiesterase Inhibitors in Asthma ph osphodiesterase inhibitors. Phosphodiesterase Bronchial asthma is a noninfecti ous respiratory (POE) inhibitors result in enhanced intracellu lar x12 pathology invo lving the concerted actions of multiple levels of second messengers cAMP and cGMp . , in flammatory cell s, spasmogens, infl ammatory which are generall y associated with dampening GUPTA e/ al.: SELECTIVE PHOSPHODIESTERAS E 4 I HIBITORS 539 effects on airway smooth mu scl e. cAMP suppresses of selec ti ve ph osph odi esterase 4 inhibitors. First, the ac ti vation and medi ator release from th ere is general conviction th at th e du al anti­ infl ammatory cell s' 2.'8.,'J (Figure I ). These mediate infl ammatory and bronchodi latory profile of POE 4 the fun cti onal res ponses of ce ll s to a mu ltitude of inhibitors could lead to th e discovery of new agents hormones, neurot ransmitters and autoco id s with able to compete and perhaps to repl ace respect to the regul atory functi on of cell s in vo lved in corti costeroid s (ant i-inflammatory) and ~ 2 ago ni sts (bronochodil ators), the bas ic th erapeut ic agents for path ophys iology of asthma. So POE inhibitors have a th e management of as thma and second , new and theoreti cal advantage over the use of other promlsll1 g therapeutic appl ications of POE 4 anti as thmati c agents th at in hi bit th e formation or inhibitors in certain autoimmune di seases, e.g., antago ni ze the action of indi vidual mediators. rheumatoid arthritis, mu lt ipl e sc leros is and type 2 8 22 Ph osph odi esterases 3, 4 and 5 are parti cularl y di abetes . . . important with respect to targets for the development 5 20 Selective PDE 4 Inhibitors as Antiasthmatic of novel anti asthmati c agents. . The mi xed anti­ infl ammatory and bronchodil atory profil e of POE Agents inhibitors could all ow the di scovery of new agents Inhibitors of low Kill, cAMP-specific type 4 abl e to compete and rep lace cOlt icosteroid s and ~ 2 isozymes in parti cular are attrac ti ve targets as ago ni sts, whi ch so fa r represent the basis of 8 potenti al anti asthmati c agents. This isozyme exerts a therapeuti c management of asthma . All five (1-5) key role in regul ati on of infl ammatory cell s major types of isozymes are found in human airways. 4 10 impli cated in the pathology of the di sease because The ro le of POE I and POE 2 isozymes have not been of its tissue di stribution. POE 4 is also an important clarified due to lac k of potent and selecti ve inh ibitors 23 of these two isozymes). Much of th e emph as is on enzy me in human bronchi . They inhibit superoxide selec ti ve POE inhibitors for as thma therapy has been generation in monocytes, , neut ro ph il s foc used on POE 4, whi ch is a cAMP specific isozyme and eosinophi ls, reduction of TNFa release in with a Kill of 3 jJ.M for cAMP and more th an 3,000 monocytes and macrop hages and su ppression of 5 11M fo r cGMP21. There are atleast two main reasons chemotax is and phagocy tosis' ,2.J. Inhibition of fo r the bas is of growing interest in the chemical, medi ator sy nthesis or release from mast cell s, ph armaco logical and bi ochemical research in the area basophils, , neutrophi ls, mac rophages and

Cyclic AMP Cyclic GMP

Adenylyl Guanylate Cyclase Cyclase ! ! ATP ~ cAMP ~ S'-AMP S'-GMP +--- cGMP +--- GTP

cA-PK ! cA-PK cG-PK ! cG-PK (inactive) ~ (active) (active) +--- (inactive) }/"~ }/" r------~ ~------~ t INFLAMMATORY CELL ACTIVITY I I t AIRWAY SMOOTH MUSCLE TONE I

Fi gure 1- Role o r cycl ic nucleotides in th e functi on of inflammatory cell s and airway smooth muscle 540 J SC I INO RES VOL 62 JUNE 2003

T- Iymphocytes have also been demonstrated. PDE 4 thi s acti vity is coexpressed with hu man recombi nant inhi bitors have th e ability to redu ce th e PDE 4 ac ti vit y. The bronchodilatory effec t of several bronoc hospasm induced by hi stamine, leukotri ene PDE 4 inhibitors correlate better with di splacement oj 8 25 18 2Y D ~( L TD4), carbac hol an d meth ac holine . Selective ro lipram binding than with POE 4 inhi bition . . In in hib it ors of PDE 4 may address not onl y asthmati c addition to hav in g desirabl e in hibitory effec ts on bronchoconslricti on bu t also underl yin g bronchi al inflammatory, anaph ylax is and smooth muscle infl ammati on.26 Thu s the pro fil e of selecti ve PDE 4 contrac ti on, selecti ve POE 4 in hibitors also prod uce inhi bitors seems to fulfill the requirements for a undes irable side effects inclu ding nausea and vomiting. unique th erapy fo r asthma. POE 4 inhibitors also potenti all y disp lace e H] roli pram fro m a hi gh affinity binding site which is pro posed to From a structu ra l po in t of view, selecti ve PDE 4 be an all osteric binding site on POE 4 enzy me. Emeti c inh ib ito rs can be divid ed into 4 cl asses. potency of PDE 4 inhibitors is corre lated with affinity Catec hol eth ers: Struc tural analogues of rolipram fo r rolipram binding site in brain rat her than potency of (1). inhibiting PDE 4 enzyme ac ti vity . Also th e mos t 2 Heterocyclics and anal ogues: Structural obvious concern regarding the POE 4 inhibitors stems analogues of nitraq uazone (2). from th eir antidepressant activ ity re 'ulting fro m their 1 and related co mpounds Structural inhibition of PDE 4 in the brain . Efforts have been analogues of (3). made to eliminate the emetic potent ia l of PDE 4 4 Mi scell aneous Selecti ve PDE 4 Inhibitors inhibitors by developing compounds with decreased [,H) rolipram binding affinit y whit retaining POE 4 D potenc/ .

Rolipram Binding Site

The hi gh affinity rolipram binding site IS an o intriguing factor in th e pu zzle of POE 4 ac ti vity regulation. It has been reported that hi gh affinity binding sites in th e rat brain membrane are labeled by (1 ) [,H) rolipram in a stereospecifi c and stereoselecti ve and saturable manner"". It has al 0 been di scl osed that a monocyte derived recombi nan t PDE 4 has a binding site with an affi ni ty for ro lipram approximately 100 times that of catalyti c site'·28. Studies with other PDE inhibitors th at bind to catalytic domain ind icated the di sti ncti on of rolipram binding site. Recentl y, ev idence indicatin g that roilpram binding site may not be allosteri c but a part of a different conformer of POE 4 has appeared'. (3) Rolipram has frequ entl y been used as a bas is for design of new and sUbtype specific POE 4 inhi bi tors. Co/echol Eth ers RO 20-1724, (4),2 is another potent compound Rolipram (1) is the prototype of thi s class of containing the catechol ether moiety fo und in selective PDE 4 inhibitors. Rolipram ori ginall y rolipram. developed, as an antidepressant is now the most stu died of all selecti ve PDE 4 inhibitors. SAR studies of a seri es of rolipram analogues as PDE inhibitors have been published). Rolipram has been found to bind to a hi gh affinity site on POE 4, di stinct from catalytic site27 . While hi gh affin ity rolipram binding site do not appear to be present in all ti ssues that contain PDE 4, (4) GUPTA el al.: SELECTIVE PHOSPHODIESTERASE 4 INH IBITORS 54 1

Several N-heterocyclic benzamide derivatives 5, moiety producing compounds that potently inhibited designed from rolipram have been demonstrated to the activation of inflammatory cells ill vitro' i. A show exceptional potency in hi stamine induced series of conformationall y constrained quarternary bronchospasm and POE 4 inhibition. RP7340 I (6)" is substituted derivative of COP-840, a potent a potent benzamide POE 4 inhibitor. It shows and selective POE 4 inhibitor was a lso synthesized by nonselectivity for inhibiting POE 4 (catal yti c site) over this group's. Bioavailable and efficacious 2-methoxy­ the displacement of rolipram from its binding site. benzimidazole based POE 4 inhibitors has also been evaluated by Regan et al.' ·\6. /Het In a senes of 5-(catechol ethers)-2- HN imidazolidinones, comprisin g structural features of both rolipram (1) and RO 20-1724 (4), the o substituents on catechol-3-oxygen was primarily modified to get compound 8 and 9 (ref. 20). R1" N--(0 (5) R =Cyclopentyl .::~NH Het = 3,S-dichloro-pyrid-4-yl I CH3 0-0 CI R1 R2 R1 R2 (8) H (10) CH3 H3co-b-CONH~ 0- CH3 1::2-

(9) CH3 (11) CH3 CI h

(6) Another series containing bi , tri and tetracyclic Bacher e/ al. 34 synthesized highly potent N­ hydrocarbons at 3-alkoxy position of the same parent phenyl rolipram derivative 7 with a hydrophilic structure has also been studied; Compound 10 and 11 substitution in 5 position of central phenyl ring. 2o were the most potent inhibitors in this series . Tetrahydro-pyrimidinone 12 was a lso prepared and was observed that two enantiomers were equipotent. POA-64 L (13) was equipotent to rolipram as an inhibitor of dog tracheali s POE 4.

(7)

Another group replaced 3-methoxy-4- cyclopentyloxy or catechol like moiety with an indole 542 J SCI INO RES VOL 62 JUNE 2003

which showed (IC50 value of 300 and 5 nm respectively for POE 4 and H ARBS), suggesting it to display an improved therapeutic acti vity with low ~o incidence of side effects as compared to rolipram.

o I [Yo COOH CH3 (13) o A novel senes of 4-(3-alkoxy-4-methoxy­ I phenyl)benzoic acids and their corresponding CH3 carboxamides have been prepared and evalu ated in an (15) effort to reduce emetic side effects' O Phenylpentoxy derivative (14) was found to exhibit potent POE 4 A seri es of rol ipram derivati ves differently inhibitory activity and possessed approximate ly 400 substituted e ither at th e pyrrolidinone or at aromatic 1X times weaker activity than rolipram for CH] rolipram ring have been synthesized and reporred . A WO-12- binding sit e and demonstrated a significant reduction 28 1 (Loteprednol), a 5-hydroxyindole derivative in emetic side effects. which has been developed by Cell tech has also been evaluated in passively sensiti zed human airways and prevented contraction in reduc in g allergen chall e nge 3 in sensiti zed guinea pig and Brown Norway rats .1') , (POE 4 inhibitor) acts by down regul ati ng-tumour necrosis factor and has shown potent anti-inflammatory activity ill vitro studies usin g novel human whole blood assay. A novel seri es of 2,2,-disubstituted indan-I ,3- dione (16) based cycli c compounds a . COOH (14) CH3 I Similar strategy led to the development of a o potent second generation inhibitor of POE 4 with a decreased potential for side effects. SB, 207499 (4-cyano-4-[3-( cyclopenty lox y )-4-methox ypheny 1]­ cyclohexane- I-carboxyli c acid), ArifloTM (15)37 o developed by Smithkline Beehcam, is presentl y undergoing phase II and III clinical tri als with respect to paediatric pati ents with asthma and patients with chroni c obstructive pulmonary disease, respective ly. o It has a large therapeutic potential and a decreased potential for side effects as compared to rolipram. It R has been suggested that agents displaying potent catalyti c site activity with a reduced activity at (16) HARBS (hi gh affinity rol ipram binding site) would have good anti-inflammatory property with decreased POE 4 inhibitors have been described . T hi s potential to induce side effects and it has been structurally unique c lass of POE 4 inhibitors has establi shed th at ArifloTM displ ays potent selective conformationally constrained indan ring linker and affi nities for POE 4 cata lytic site (lCso = 95 nm) and led to the identification of inhibitors with nanomolar for HARBS CICso = 120 nm) compared to rolipram potency, no emetic and oral activity" . GUPTA e/ {I I.: SELECTIVE PHOSPHODIESTERAS E 4 INHIBITORS 543

Heterocyclics and Analogues o This class of PDE 4 inhibitors is exemplified by nitraquazone (2). The archetypical quinazolindione moiety of compound 2 has been extensive ly manipulated to afford a vari ety of structure derived compounds. 3'-N02 group of compound 2 has been replaced by different nonprotic, e lectron-withdrawing 8 functionalities li ke -CI, -Br, -COOCH3 to study SAR . Corresponding acid and N-methyl amide produced a substantial and complete loss . In potency (23) respectl. vel y ~ ,?-O . Pyridopyrimidinedione ana logues have also These compounds showed significantl y better been prepared. A lthough benzene-pyridine isosteric balance between PDE 4 inhibition and emeti c side replacement led to decrease in potency in case of effects. Further simplificatio n led to quinoline compound 17 as compared to compound 18 but deri vati ve RS 14203 (24) wh ich is one of the most potent PDE 4 inhibitor. introducti on of bulkier groups at N3 afforded 19-21 with substantially in creased potency. The corresponding 4-pyridyl derivative 22 has proved to be four times potent inhibitor of PDE 4 wi th respect 40 to prototype 17 . Compounds of the type 23 containing pyridopyridazinone nucleus are nanomolar selecti ve inhibitors of PDE 441 . The concomitant increase in side effects limited their development. Attempts were made to reduce these adverse effects and compounds were synthesized by replacing pyridine w ith a seri es of heterocyclic systems like pyrrole, pyrazole, 1,2-dihydropyridine and (24) th iophene42.43 . o

R1 (25) X R1 R2 (17) N COOCH3 CH3 Rep Iaceme n t of 3' nitrophenyl group of (18) CH COOCH3 CH3 compound 24 by benzotriazole resulted in formati on of compound 25. (19) N COOCH3 phenyl Naftiridinones 26 and compound 27 (ref. 8 (20) N COOCH3 cyclopentyl and 44) result from a lesser structural simpli ficati on (21 ) N COOCH3 norbornyl of nitraquazone. Heterocycli c fused 3[2H]-4 pyridazinones 28 have been synthesized and reported (22) N N02 4-pyridyl to have potent selective PDE 4 inhibitory activity and 544 J SCI IND RES VOL 62 JUNE 2003 greatly attenuated affinity for rolipram high affinity A novel class of inhi bitors like compound 31 bIn· dIIl' g sit. e41· . and 32 based on same guinoline. guinolone and naftiridinone template but also with the presence of carboxamido or sulfonamido groups have been 48A9 N::::"\ reported . N R cC:x _ N 0 N N 0 )0 R, R1 = cycloalkyl F 6 X= N, CH, CR1 (26) (27) (31 ) /RS 0 0 X-N ""- c('NHM R6 N a R

Gtz X= co, cs (32) (28) (29) Nitraguazone like inhibitors based on an almost Nicotinamide ethers 29, ring opened variants of flat heteroaromatic area generally formed by 6-6 pyridopyridindiones have recentl y aroused renewed condensed system; One (hetero) aromatic or a interest in inhibition of POE 4. They also have cycloalkyl systems connected to flat portion through a methylene spacer with an electron withdrawina reduced e metic sid e effects. These compounds were . R b 45 substituent represents the best pharmLlcophore model earli er reported in 199 1 . A variety of substituents Z = -F, -C!, -OCH" -CN, -CF, have been prepared. 33. Chall enge in this c lass of POE 4 inhibitors is to Meta and para positions were found to be favourable ha ve th e selective PDE 4 inhi bitory activity in . 4) b ut or(1I o was not favoured · . Recently, POE 4 nanomolar potency with strongly reduced affinity for inhibitors based on pyrido[2,3-dJpyrazino 30 high affinity rolipram binding site to improve the 46 therapeutic index. backbone have been disclosed ,47. ~0 N X

~ Y (30) (33) GUPTA el al.: SELECTIVE PHOSPHODIESTERASE 4 INHI BITORS 545

A new type of bronchodilator agents, bi- and F rom the common phannacoph ore for tri-cycli c nitrogen bridgehead compounds w it h a nitraquazone related compounds, a seri es of novel pyrimidine-4(3H)-one ring have been synthesized heteroaromatic compounds have been designed, and evalu ated for bronchodilatory activity. One of synthesized and evalu ated as PDE 4 inhibitors. th e compounds was found as a potent Thienopyrimidin (37) was selected a ~; a lead 50 bronc hodi lator . compound and a number of compounds with vari ous A seri es of novel tricyclic heterocyclics li pophilic and aromatic groups in both C-2 and C-4 imidazo[4,5-c] [1 ,8] naphthyridin-4-(5H)-ones have positi on were synthesized. These compounds showed been designed and synthesized4-l. Compound 34 a good balance of PDE 4 activity and displcelllent of 52 relaxed spontaneous guinea pig isolated tracheal [.\ H] rolipram from its binding site . preparation with 4- 16 fold greater potency than Order of reacti vity of pteri dine carbon atoms ami nophylline. It also inhibited PDE 4 towards secondary amines have been elu c id ated and /H alkylamino substituted pteridines 38 free of positi onal HN . I b I" \ Isomers lave een preparec .. .

ro::--...'N N 0 C) N CI N:/" """ 6 /\ ~ I h (34) (35) L--.I H I ~ New heterocyclic compounds 3H-imidazo[4,5- c]quin azoline-4(5H)-ones 35 have been designed by (38) Suzuki ef a l .51 This tricycli c heterocycli c can be regarded as a fu sion compound of a benzene ring to o th e bond between 2 and 3 position of 7-substituted . 5-Ethyl-3-methyl derivative was found to be OH 5-fold more active than theophylline for their protecti ve effect against antigen induced contraction of guinea pig trachea. Similarly, 7,5-disubstituted I H-imidazo[4 ,5- c]quinolin-4[5H]-ones (36) were synthesized and were tested as potent and active bronchodil ators. The most potent compound of this seri es showed weak POE 4 inhibition whi ch could not account for it s potent bronchodi lati on. N~CH3 'S---yN (39)

6,8-0isubstitutd-I ,7-naphthyridines (NVP-AB E HN6 171 , 39 ), a new class of selective phosphodiesterase I" inhibitors has recently been reported 54 . 39 Has been .--::: found about 40-times more potent as compared to Ariflo (15). A seri es of l-aryl-2,3-bi(hydroxymeth yl) (36) (37) naphthalene li gnan have been synthesized and 546 J SCI IND RES VOL ('2 JUNE 2003 evalu ated fo r their ability to selectively inhibit PDE 4 cause a variety of physiological effects. The CNS in isolated pi g tracheal strip. Replacement of stimulatory properties of have been utili zed I-phenyl ring by a pyridone ring led to marked for centuries. Tachycardia and bronchodilation are improvement of their selectivity for . PDE 4 over other responses el icited by xanthines. Inhibition of PDE 3. Compound 40 was chosen as candidate for phosphodiesterase in heart, brain and lungs may be s5 further pharmacological evaluation . the mechanism by which xanthines exert thei r 57 A new approach to improve therapeutic window effects . Theophylline preparations have found of PDE 4 inhibitors is aimed at the identification of continuous use as bronchodilators in the treatment of th e specific targets for emesis and efficacy. An asthma for almost a century. It is a weak non­ emetic, efficacious and competitive PDE 4 inhibitor selective phosphodiesterase inhibitor, but it is 41 capable of covalently tagging its biological targets bel ieved that PDE inhibitory activity may contribute to both its bronchodilatory and anti-inflammatory upon photoactivation has been synthesized. Highly 58 emetic and efficacious PDE 4 photoaffinity probe has activities . Although theophylline is useful in the been reported. This probe would be highly useful for treatment of asthma, the value of theophylline is id entification of respective targets through which limited to a narrow therapeutic index due to wide range of gastrointestinal, CNS and cardiovascular PDE 4 specific inhibitors cause emesis and also 59 6 side effects . The drugs that couple the efficacy of produce their efficac/ . theophylline with an improved side effects profile and an increased therapeutic index would be an important

OH advance in the treatment of asthma. Attempts have OH been made to improve therapeutic profil e by synthesizing new xanthine analogues without specifically focusing on PDE inhi bitory activity but so far these efforts have not been successful. An alternative approach towards developing an 'improved theophylline' has emerged recently. Using xanthine skeleton, attempts have been made to design and synthesize novel compounds which are selective PDE 4 inhibitors and also retain tt he therapeutic efficacy of theophylline. Xanthine nucleu present in theophylline is also present in the PDE substrate and (40) will bind to and block highly conserved enzyme 60 catalytic site. Miyamoto et al. ,6 J studied SAR of this class and reported an interesting series of heterocyclic fu sed xanthines. A series of xanthines with varied substituents at the 1,3 and 8 positions have been prepared in order to understand the SAR for alkyl xanthines as inhibitors 8 of phosphodiesterase ,62.

Many 7-alkylated derivatives of theophylline such as dyphylline (42), doxophylline (43), thioanalogue of doxophylline (44), (41 ) (45), bamiphylline (46), acephylline (47) and etophylline (48) have been synthesized and 5 studied ,6J. Among the therapeutically useful Xanthine and Related Compounds synthetic xanthine analogues, dyphylline (42) has Theophylline represents this class of selective been approved and has appeared in the US market as PDE 4 inhibitors. Xanthines have long been known to an antiasthmatic drug. Bamiphylline (46) and GUPTA el al.: SELECTIVE PHOSPHODIESTERASE 4 INH IBITORS 547

5 Doxophylline (43) have been marketed in Europe ,62. It is 25-30 fold less emetic as compared to These deri vatives are generall y less active and exhibit rolipram. demonstrated significant low or moderate PDE 4 inhibitory activity than improvement of pulmonary funct ion, as well as safe theophylline but are stable in solution and in vivo. profile. Structural analogues of arophylline have been T hese possess same side effects as theophylline. reported by a chiroscience group. T he compounds o showed improved ratio for PDE 4 inhibitory activity versus emetic side effects. Cipamphylline (51) (BRL 60063) was the prototype of a series of potent PDE 4 H3C-i~~>-R' inhibitors reported and has been synthesized by 66 OANJlN Smithkline Beecham . Sulfonation of c ipamphylline I at 7- and 8-positions increased potency against PDE CH3 5A as compared to PDE 4 isozymes. 8-substituted R1 R2 piperazine derivatives have been reported by Ragnier (42) CH2CH(OH)CH20H H et at. 67 having a combination of antiall ergic and o antihistaminic properties. They also di splayed a potent bronchodilatory activity with enhanced (43) CH2 -< J H o duration of action. Among these, l-methyl-3-isobutyl-8- S (4-benzhydrylpi perazinoethyl) xanthine S9795 (52) exhibited inhibitory action on mast cell degranulation (44) CH2 J H 8 67 -< . . and phosphodiesterase enzyme s (45) CH2CH(OH)CH3 H

(46) ICH,I, - NIC,HsHCH,l,OH CH,--o

(47) CH2COOH H

(48) CH2CH20H H

Denbufylline (49) is a selecti ve PDE 4 inhibitor showin g low adenosine receptor affinitl'64, but the development of compound was discontinued because (51 ) of poor pharmacokinetics. Arofylline (50) is in phase ill clinical trials for oral asthma th erap/5. It is a weak but selective inhibitor of PDE 4 as compared to theophylline.

(52)

A series of benzoseparated compounds purines, R2 R3 isoguanidines and thioxanthines whi ch inhibit PDE 4 CH2CH(OH)CH20H CH3 from bovi ne tracheal smooth muscle have been synthesized and reported.68 Depending upon the (50) pC1CsH4 substituents at C-7, C-8 or C- I positions, these 548 J SCI IND RES VOL 62 JUNE 2003

benzoseparated linear deri vatives of 3-isobutyl-l­ Illethylxanthine 53 show POE inhibiting activity.

o R" I 1 N

R Ir '" N

(53)

(57) (58)

These compounds have progressed into phase If clinical tri als for asthma.

Miscellaneous Selective PDE 4 Inh ibitors POE 4 inhibitors based on benzofuran (59) and (54) benzopyran (60) nucleus have been reponeds. (61) One of the xanthine analogue, (54), Showed POE 4 inhibitory activity in the nanomolar marketed as an oral ly actin g antiasthmatic druo in ranges. Indazole (62)74 and pyrazolo[3,4-c]pyridine H.W . .. b (63) represent varioLls other POE 4 inhibitors Japan has been found to be a nonspecIfIc and 7 moderate ly potent POE 4 inhibitor. synthesized by Pfizer Labs ) . SCA 40 (55)70 is an effective bronchodilator in CH3 human bronchus precontracted with different / spasmogens. The presence of bromjne on position 6 o and an amino (alkylamino) group on positi on 8 plays a critical role in the activity. It also shows POE inhibitory activity in micromolar range. ICI-63197 (55) is another reported compound which is a weak selecti ve inhibitor of POE 4 71. R (59)

NHCH3 CH / 3 o NJy}(N CH3 ~ 0 /CH:I ~N o Br 0.. I CH3

(56) (55) CIUINH~ 0 CI Efforts have been made to prepare hybrid I" structures of xanthine skeleton and rolipram. V 11294 ,0 N A (57)72 and RPR 132703 (58)73 are the most He! interesting examples. (60) (61) GUPTA et al.: SELECTIVE PHOS PHOD IESTERASE 4 INHIB ITORS 549

Conclusions CI N '" With the development of selecti ve POE 4 inhibitors, interest and excitement has been generated I ~ N -C /J to discover new cl asses of drugs fo r th e treatment of H " VCI 0 asthma. Clinical experi ence with the use of isozymes selective POE 4 inhibitors for the treatment of as th ma is limited. Determination of the utility of this novel (62) strategy currently bein g evalu ated for asthma require additional clinical studies.Although mechani sm of acti on, safety and continuous efficacy of POE 4 inhibitors on chronic use is still questi onabl e,it seems to be reasonable isozyme fo r continued evalu ati on as novel bronchodi lators. Evidence is also mounting in support of POE 4 as a target for anti-inflammatory agents. Combining POE 4 inhibitory acti vity with POE 3 or POE 5 inhibitory acti vity within a single molecule could lead to a very effective bronchodilator with anti-inflammatory acti vity, although there wi ll (63) also be increase in side effects. Benzodiazepine deri vati ves (64)76 represent No selecti ve POE 4 inhibitor is currently novel POE 4 inhibitors with potent bronchodilatory marketed. Some of the selecti ve POE 4 inhibitors are acti vity. Recentl y, a novel seri es of pyridazin ones and in phase II and III clinical trials as anti asthmati c phthazin ones (65) have also been reported as potent agents. The recent discovery of two POE 4 and selecti ve inhibitors of POE 4 10. conformers HPOE4 and LPOE4 with their unique ti ssue di stribution and activity has further contributed to design of new and specific inhibitors wi th decreased side effects. It has been accepted th at hi gh affinity for HARBS is associated with more NH -c Het propensity to induce side effects. Thus hi gh potency R II o at catalytic site and reduced affinity for HARBS wi th selectivity for POE 4 may afford the ideal physiological and toxicological profil e for drugs wi th clinical efficacy in asthma. One such compound is SB207499 (ArifioTM) which is in the final stages of clinical trials. Several POE 4 inhibitors based on the (64) modification of theophylline, rolipram or nitraquazone are currently undergoing evaluation and show much promi se for future therapy of asthma. Although the POE 4 inhibitory effect of therapeutically useful antiasthmati c drugs, methylxanthines was initiall y considered as an important mechanism in their antiasthmati c acti vi ty, but they generall y di splay lower or moderate potency at the catalyti c site e.g. arofylline, a POE 4 inhibitor, is one of the most advanced drugs in clinical (65) evaluati on. The mechani sm remain s controversial and 550 J SCI IND RES VOL 62 JU E 2003 data suggest that for clinical efficacy in asthma N, Terrasse C, Wri ggl eworth R & D berty A M, Synth es is, Structure -acti vity relati onshi ps , and pharmaco logica l pro fil e potency at cata lytic site of PDE 4 inhibitors is not an of 9-amino-4-oxo-l -phenyl-3,4,6,7-tetrahydro[ I ,4]di azepin o essential requirement. To prove this, further, [6,7,I-hi ]indoles: Di scovery of potent. selec ti ve Phosphod i­ development of many potent PDE 4 inhibitors have esterase 4 inhi bitors, J Med Chelll , 43 (2000) 4850. been discontinued as antiasthmatic agents in recent 7 Cavall a 0 & Frith R, Phos ph odi esterase IV In hibit ors: years. Sti II there is much promise remaining in the Structu ra l Di versit y and Therapeutic Potent ia l in Asth ma, research in the field. CUlT Med Chelll, 2 (1995 ) 561. 8 Dal Pi az V & Giovannoni M p, Phos phodiesterase 4 Theophylline nucleus may be very useful for inhibi to rs, stru cturall y unrel ated to Rolipram, as promisin g desi gning new antiasthmatic drugs with inherent agent s for the treatment of asthma and other pa th ologies , EliI' antiasthmatic properties of this old drug and .I Med Chelll, 35 (2000) 463 . decreased cardiovascular and CNS side effects. On 9 Beavo J A, Cycli c nucleotid e Phosphodiesterases: Fu nct ional the whole, selective PDE 4 inhi bitors, particularly implicati ons of multipl e forms, Physiul Rev, 75 (1995) 725. xanthines open a very interesting prospective of a 10 Mey M V, Hatzelmann A, Laan I J V, Sterk G J. Thi ba ut U breakthrough in this area of research for medicinal & Timmerman H, Novel selecti ve PD E4 inhi bit ors. I. chemi sts . The rational design of PDE 4' inhibitors has Synthesis stru cture-acti vit y rel ati onships, and molecu lar taken on new meaning and vitality and now selective modeling of 4-(3,4-Dimeth oxyphenyl)-2 H-phthalaz in-I-ones and an alogues, .I Med Chelll . 44 (200 I) 251 I . PDE 4 inhibitors could be designed with a specific II Whelan C J, Pros pec ts for th e development of new dru gs for th erapeutic endpoint for optimum clinical benefit. the th erapy of respiratory di seases, IJl'IIgs Tuday 32 ( 1996) Acknowledgements 295. 12 Raeburn 0 & Karl sson J A, Phosphodi es terase inhi bito rs: Th is re vi ew is the outcome of literature survey kn ckout drops?, J Pharlll Pham lClcol. 49 (1997) 19. fo r th e research project entitled " De velopment of 13 McFadden Jr. E R & Gilbert I A, Asthma, N Ell gi J Med, 327 antiasthmatic agents" financed by Haryana State ( 1992) 1928. Counc il for Sc ience and Technology, Chandigarh, 14 Barnes P J, New drugs for asthma, Eli I' Re.lpir.l, 5 ( 1992) India. 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