John E. Souness

Proposal for Pharmacologically Distinct Conformers of PDE4 Cyclic AMP Phosphodiesterases

cAMP-specific phosphodiesterase inhibitors display a range of activities in vitro and in vivo which suggest they may be useful in the treatment of inflammatory diseases. However, these compounds elicit a number of side-effects which may limit their therapeutic potential. Certain side-effects of PDE4 inhibitors such as emesis and gastric acid secretion are associated with their actions at a high affinity rolipram binding site (HARBS). In contrast, a number of anti-inflammatory actions of PDE4 inhibitors are better correlated with inhibition of PDE4 catalytic activity than with displacement of [3H] rolipram from HARBS. This suggests that native PDE4s in different cell-types can be discriminated pharmacologically. Although known to be associated with PDE4, the nature of HARBS is uncertain. The majority of evidence suggests it represents particular conformational states of PDE subtypes with which rolipram interacts with high potency (KD approximately 2 nM) (High-affinity PDE4, HPDE4). Rolipram is generally moderately or weakly active (IC50-200 nM-2000 nM) in inhibiting catalytic activity of the majority of crude, partially-purified or recombinant PDE4-preparations (Low-affinity PDE4, LPDE4). Solubilization or V/GSH treatment of particulate eosinophil PDE4, cAMP-dependent kinase activation of RNPDE4D3 and membrane association of HSPDE4A4 increase the potencies of some (e.g., rolipram) but not other (e.g., trequinsin) inhibitors. In eosinophils, the changes in enzyme properties brought about by solubilization result in a close correlation between the potency order of compounds in inhibiting cAMP hydrolysis and displacing [3H] rolipram from HARBS. The identification of distinct pharmacological PDE4 forms may have therapeutic consequences since it may be possible to synthesize potent inhibitors of LPDE4 with low affinity for HARBS which should, theoretically, be less emetic. Most inhibitors synthesized to date (rolipram, denbufylline nitraquazone, etc.) display high-affinity for HARBS but are much weaker in inhibiting cAMP hydrolysis. Other compounds (RP 73401, trequinsin, CDP 840) display slightly higher potency against LPDE4 or do not discriminate between the two putative PDE4 forms. Recently, inhibitors have been synthesized which are considerably more active against LPDE4 than HPDE4. Such compounds with appropriate pharmacokinetic properties may retain anti-inflammatory activity but have a reduced capacity to cause nausea and emesis and, consequently, have a wider therapeutic window than compounds currently undergoing clincial evaluation.

Anti-inflammatory and bronchodilator properties of RP 73401, a novel and selective phosphodiesterase type IV inhibitor.

1 We have investigated the effects of RP 73401, a novel, potent and highly selective cyclic nucleotide phosphodiesterase (PDE) type IV inhibitor, in guinea‐pig and rat models of bronchoconstriction and allergic inflammation. In some models, the effects of RP 73401 have been compared with those of the standard PDE type IV inhibitor, rolipram. 2 RP 73401 (0.4–400 μg kg−1, intratracheally (i.t.) on lactose) inhibited antigen‐induced bronchospasm in previously sensitized conscious guinea‐pigs (ID50: 7 ± 1 μg kg−1) and in anaesthetized rats (ID50: 100 ± 25 μg kg−1). Rolipram inhibited the antigen‐induced bronchospasm in guinea‐pigs with an ID50 of 5 ± 1 μg kg−1. In guinea‐pig bronchoalveolar lavage (BAL) fluid, total inflammatory cell and eosinophil numbers were reduced by RP 73401 (ID50s: 3.9 ± 0.8 μg kg−1 and 3.2 ± 0.7 μg kg−1, respectively). In the rat, inflammatory cell numbers are less affected. Only the highest dose of RP 73401 (400 μg kg−1) significantly inhibited eosinophil influx (41 ± 16% inhibition). 3 RP 73401 (0.02–100 μg kg−1, i.v.) inhibited PAF‐induced bronchial hyperreactivity to bombesin in the anaesthetized guinea‐pig (ID50: 0.09 ± 0.03 μg kg−1) and inhibited (0.4–40 μg kg−1, i.t.) histamine‐induced airway microvascular leakage in the anaesthetized guinea‐pig by approximately 60% at all doses. 4 RP 73401 relaxed guinea‐pig isolated trachea under basal tone (EC50: 9 nm) and when precontracted with histamine (IC50: 2 nm), methacholine (IC50: 29 nm) or leukotriene D4 (LTD4, IC50: 4 nm). 5 RP 73401 (0.4–100 μg kg−1, i.t.) inhibited bronchospasm induced by histamine (ID50: 34 ± 6 μg kg−1), methacholine (ID50: 66 ± 12 μg kg−1) and LTD4 (ID50: < 4 μg kg−1) in the anaesthetized guinea‐pig. Against these same bronchoconstrictors, rolipram (i.t.) had ID50 values of 44 ± 4, 72 ± 18 and < 4 μg kg−1 respectively. RP 73401 (4 and 40 μg kg−1, i.t.) increased the magnitude and duration of bronchodilatation produced by salbutamol in the anaesthetized guinea‐pig. At doses producing significant bronchodilatation, RP 73401 was without effect on heart rate or blood pressure in the anaesthetized guinea‐pig. RP 73401 (0.01‐0.25 mg kg−1, i.v.) did not affect heart rate and produced only a small fall in blood pressure in the anaesthetized rat. 6 These data demonstrate that RP 73401 and rolipram inhibit antigen‐ and mediator‐induced bronchospasm in guinea‐pigs with the same potency. Furthermore, RP 73401 administered directly into the airways, protects against allergic airway inflammation. These results indicate the importance of PDE IV in regulating smooth muscle and inflammatory cell activity. At doses suppressing the inflammatory response in the lung, RP 73401 had little effect in the cardiovascular system. RP 73401 may have a role as a bronchodilator and, more importantly, as a prophylactic anti‐inflammatory agent in the treatment of asthma.

Evidence that cyclic AMP phosphodiesterase inhibitors suppress TNFα generation from human monocytes by interacting with a ‘low‐affinity’ phosphodiesterase 4 conformer

1 We have investigated the inhibitory effects of RP 73401 (piclamilast) and rolipram against human monocyte cyclic AMP‐specific phosphodiesterase (PDE4) in relation to their effects on prostaglandin (PG)E2‐induced cyclic AMP accumulation and lipopolysaccharide (LPS)‐induced TNFα production and TNFα mRNA expression. 2 PDE4 was found to be the predominant PDE isoenzyme in the cytosolic fraction of human monocytes. Cyclic GMP‐inhibited PDE (PDE3) was also detected in the cytosolic and particulate fractions. Reverse transcription polymerase chain reaction (RT‐PCR) of human monocyte poly (A+) mRNA revealed amplified products corresponding to PDE4 subtypes A and B of which the former was most highly expressed. A faint band corresponding in size to PDE4D was also observed. 3 RP 73401 was a potent inhibitor of cytosolic PDE4 (IC50: 1.5 ± 0.6 nM, n = 3). (±)‐Rolipram (IC50: 313 ± 6.7 nM, n = 3) was at least 200 fold less potent than RP 73401. R‐(−)−rolipram was approximately 3 fold more potent than S‐(+)‐rolipram against cytosolic PDE4. 4 RP 73401 (IC50: 9.2 ± 2.1 nM, n = 6) was over 50 fold more potent than (±)‐rolipram (IC50: 503 ± 134 nM, n = 6)) in potentiating PGE2‐induced cyclic AMP accumulation. R‐(−)−rolipram (IC50: 289 ± 121 nM, n = 5) was 4.7 fold more potent than its S‐(+)‐enantiomer (IC50: 1356 ± 314 nM, n = 5). A strong and highly‐significant, linear correlation (r = 0.95, P < 0.01, n = 13) was observed between the inhibitory potencies of a range of structurally distinct PDE4 inhibitors against monocyte PDE4 and their ED50 values in enhancing monocyte cyclic AMP accumulation. A poorer, though still significant, linear correlation (r = 0.67, P < 0.01, n = 13) was observed between the potencies of the same compounds in potentiating PGE2‐induced monocyte cyclic AMP accumulation and their abilities to displace [3H]‐rolipram binding to brain membranes. 5 RP 73401 (IC50: 6.9 ± 3.3 nM, n = 5) was 71 fold more potent than (±)‐rolipram (IC50: 490 ± 260 nM, n = 4) in inhibiting LPS‐induced TNFα release from monocytes. R‐(−)−rolipram (IC50: 397 ± 178 nM, n = 3) was 5.2‐fold more potent than its S‐(+)‐ enantiomer (IC50: 2067 ± 659 nM, n = 3). As with cyclic AMP, accumulation a closer, linear correlation existed between the potency of structurally distinct compounds in suppressing TNFα with PDE4 inhibition (r = 0.93, P < 0.01, n = 13) than with displacement of [3H]‐rolipram binding (r = 0.65, P < 0.01, n = 13). 6 RP 73401 (IC50: 2 nM) was 180 fold more potent than rolipram (IC50: 360 nM) in suppressing LPS (10 ng ml−1)‐induced TNFα mRNA. 7 The results demonstrate that RP 73401 is a very potent inhibitor of TNFα release from human monocytes suggesting that it may have therapeutic potential in the many pathological conditions associated with over‐production of this pro‐inflammatory cytokine. Furthermore, PDE inhibitor actions on functional responses are better correlated with inhibition of PDE4 catalytic activity than displacement of [3H]‐rolipram from its high‐affinity binding site, suggesting that the native PDE4 in human monocytes exists predominantly in a ‘low‐affinity’ state.

Characterization of guinea-pig eosinophil phosphodiesterase activity: Assessment of its involvement in regulating superoxide generation

Experiments have been performed to characterize guinea-pig peritoneal eosinophil cyclic nucleotide phosphodiesterase (PDE) activity and establish whether it is involved in regulating superoxide (.O2-) generation. Eosinophils were found to contain a predominantly membrane-bound cAMP PDE(s) (92.5 +/- 2.4% of total activity) which was resistant to solubilization with Triton X-100 (1%). This particulate PDE exhibited complex kinetics (Km = 1.3 and 31.4 microM) and was unaffected by cGMP (IC50 greater than 100 microM) or CaCl2 (2 mM) + calmodulin (10 units/mL). Little cGMP PDE activity was detected in either the soluble or particulate fractions. Inhibitors of the Ro-20-1724-inhibited (Type IV) cAMP PDE, namely Ro-20-1724 (IC50 = 0.92 +/- 0.43 microM), rolipram (IC50 = 0.20 +/- 0.04 microM) and denbufylline (IC50 = 0.20 +/- 0.01 microM), potently inhibited the particulate cAMP PDE, as did the non-selective inhibitors trequinsin (IC50 = 0.11 +/- 0.02 microM) and AH-21-132 (IC50 = 2.57 +/- 0.02 microM). Eosinophil cAMP PDE was resistant to SK&F 94120 (IC50 greater than 1000 microM), the cGMP-inhibited (Type III) cAMP PDE inhibitor, and the cGMP PDE (Type I) inhibitor, zaprinast, was only weakly active (IC50 = 35.33 +/- 10.74 microM). .O2- release from resting cells was potently inhibited by rolipram (IC50 = 0.05 +/- 0.03 microM) and denbufylline (IC50 = 0.06 +/- 0.04 microM) but surprisingly, in view of its potent cAMP PDE inhibitory activity, was only weakly decreased by trequinsin (IC50 = 8.0 +/- 2.7 microM). AH-21-132 (IC50 greater than 10 microM), SK&F 94120 (IC50 greater than 10 microM) and zaprinast (IC50 greater than 10 microM) were without effect. Rolipram and denbufylline alone exerted little effect on cAMP in intact cells but, in the presence of 10 microM isoprenaline, potently increased intracellular accumulation (EC50 = 0.45 +/- 0.16 and 0.28 +/- 0.08 microM, respectively). Trequinsin and AH-21-132 only weakly enhanced isoprenaline-stimulated cAMP accumulation. Although it induced a marked rise in cAMP only in the presence of isoprenaline, rolipram (50 microM) alone was able to increase the activity ratio of cAMP-dependent protein kinase from 0.24 to 0.84. The results suggest that Ro-20-1724-inhibited cAMP PDE plays a role in regulating eosinophil .O2- generation. The poor correlation between the PDE inhibitory actions of certain compounds and their effectiveness in elevating cAMP and inhibiting .O2- suggests the existence of a barrier impeding access to the enzyme.

The discovery of RPR 200765A, a p38 MAP kinase inhibitor displaying a good oral anti-arthritic efficacy

RPR132331, a 2-(2-dioxanyl)imidazole, was identified as an inhibitor of tumour necrosis factor (TNF)alpha release from lipopolysaccharide (LPS)-stimulated human monocytes. An intensive programme of work exploring the biology, toxicity and physical chemistry of a novel series of inhibitors, derived from RPR132331, has led to the identification of RPR200765A, a development candidate for the treatment of rheumatoid arthritis (RA). RPR200765A is a potent and selective inhibitor of p38 MAP kinase (IC50 = 50 nM). It inhibits LPS-stimulated TNFalpha release both in vitro, from human monocytes (EC50 = 110 nM), and in vivo in Balb/c mice (ED50 = 6 mg/kg). At oral doses between 10 and 30 mg/kg/day it reduces the incidence and progression in the rat streptococcal cell wall (SCW) arthritis model when administered in either prophylactic or therapeutic dosing regimens. The compound, which is a mesylate salt and exists as a stable monohydrate, shows good oral bioavailabiltiy (F = 50% in the rat) and excellent chemical stability. The data from the SCW disease model suggests that RPR200765A could exhibit a profile of disease modifying activity in rheumatoid arthritis (RA) patients which is not observed with current drug therapies.

Inhibition of pig aortic smooth muscle cell DNA synthesis by selective type III and type IV cyclic AMP phosphodiesterase inhibitors

Foetal calf serum (FCS) and platelet-derived growth factor (PDGF)-stimulated incorporation of [3H]thymidine into pig aortic smooth muscle cell (ASMC) DNA was decreased by agents that either stimulated the synthesis (forskolin) or inhibited the breakdown (3-isobutyl-1-methylxanthine, IBMX) of cAMP. FCS-stimulated incorporation of [3H]thymidine into DNA was also reduced by selective inhibitors of cAMP-specific phosphodiesterase (PDE IV) (Ro-20-1724, rolipram) and cGMP-inhibited cAMP PDE (PDE III) (SK&F 94836). IBMX, Ro-20-1724, rolipram and SK&F 94836 enhanced forskolin inhibition of DNA synthesis. Alone, rolipram was a relatively weak inhibitor of FCS-induced ASMC DNA synthesis (IC25 greater than 20 microM); however, in the presence of a threshold concentration of SK&F 94836 (20 microM), the potency of rolipram increased (IC25 = 4 microM), suggesting synergy in the actions of PDE III and PDE IV inhibitors. SK&F 94836 and rolipram elicited 30% and 37%, respectively, reductions in FCS-induced ASMC proliferation and potentiated the inhibitory actions of forskolin. PDE III and PDE IV inhibitors alone, exerted minimal effects on ASMC cAMP levels after a short term (10 min) or long-term (2 or 24 hr) exposure, but enhanced forskolin-induced accumulation of cAMP. ASMC spontaneously released cAMP into the extracellular medium, a process that was increased by forskolin. PDE III and PDE IV inhibitors had no effect alone on cAMP extrusion but enhanced the effect of forskolin. Exposure of ASMC to forskolin or SK&F 94836 for 15 min increased the activity ratio (AR) of cAMP-dependent protein kinase from 0.05 to 0.17 and 0.23, respectively. Ro-20-1724, alone, did not affect cAMP-dependent protein kinase but enhanced the stimulatory effect of forskolin (AR = 0.37) and SK&F 94836 (AR = 0.27). Agents that increased cGMP synthesis (glycerol trinitrate, atrial natriuretic factor) or decreased its hydrolysis by selectively inhibiting cGMP-specific PDE (PDE V) (zaprinast) exerted no effects on FCS- or PDGF-stimulated [3H]thymidine incorporation into DNA either alone or in combination. The cytosolic fraction of pig ASMC contained four cyclic nucleotide PDEs which were categorized as PDE V, Ca2+/calmodulin-stimulated PDE (PDE I), PDE III and PDE IV. PDE I and III activities were also associated with the particulate fraction. The results demonstrate that inhibitors of PDEs III and IV alone or in combination with forskolin, reduce ASMC DNA synthesis and proliferation, through an action likely to involve elevation of intracellular cAMP. In contrast, inhibition of cGMP hydrolysing PDE subtypes (I and V) exerted no effect on DNA synthesis in this cell type.

Role of selective cyclic GMP phosphodiesterase inhibition in the myorelaxant actions of M&B 22,948, MY-5445, vinpocetine and 1-methyl-3-isobutyl-8-(methylamino)xanthine

1 The mechanism by which M&B 22,948, MY‐5445, vinpocetine and 1‐methyl‐3‐isobutyl‐8‐(methylamino)xanthine (MIMAX), which have been described as selective cyclic GMP phosphodiesterase (PDE) inhibitors, relax rat aorta was investigated. 2 Three cyclic nucleotide PDEs were identified in the soluble fraction of rat aorta; a Ca2+‐insensitive form exhibiting substrate selectivity for cyclic GMP (cGMP PDE), a Ca2+/calmodulin‐stimulated form which also preferentially hydrolyzed cyclic GMP (Ca2+ PDE), and a form demonstrating substrate selectivity for cyclic AMP (cAMP PDE). 3 M&B 22,948 and MIMAX inhibited cGMP PDE (Ki = 0.16 μm and 0.43 μm, respectively) and Ca2+ PDE (Ki = 9.9 μm and 0.55 μm, respectively), but exhibited weak activity against cAMP PDE (Ki = 249 μm and 42 μm, respectively). MY‐5445 selectivity inhibited cGMP PDE (Ki = 1.3 μm) and vinpocetine selectively inhibited Ca2+ PDE (Ki = 14 μm). 4 M&B 22,948 and MIMAX induced dose‐dependent increases in the accumulation of cyclic GMP, but not cyclic AMP, in rat aorta pieces. These effects were greatly reduced by endothelial denudation and by methylene blue (5 μm) which blocks the actions of endothelium‐derived relaxant factor. MY‐5445 and vinpocetine had no effect on rat aorta cyclic GMP or cyclic AMP accumulation. 5 All four compounds caused dose‐related relaxation of 5‐hydroxytryptamine (10 μm) contracted, endothelium‐intact rat aorta, the effects of M&B 22,948 and MIMAX being greatly reduced by methylene blue (5 μm). Methylene blue also caused 10 fold and 100 fold rightward shifts in the dose‐response curves of MY‐5445 and vinpocetine, respectively. 6 The results are consistent with the smooth muscle relaxant actions of M&B 22,948 and MIMAX, but not vinpocetine and MY‐5445, being mediated through a mechanism involving inhibition of cyclic GMP hydrolysis.

Possible role of cyclic AMP phosphodiesterases in the actions of ibudilast on eosinophil thromboxane generation and airways smooth muscle tone

1 The possible role of cyclic AMP phosphodiesterase (PDE) in the inhibitory actions of ibudilast on tracheal smooth muscle contractility and eosinophil thromboxane generation was investigated. 2 Ibudilast was a non‐selective inhibitor of partially purified cyclic nucleotide PDE isoenzymes from pig aorta and bovine tracheal smooth muscle, exhibiting only moderate potency against bovine tracheal PDE IV (IC50 = 12 ± 4 μm, n = 3). Similar or slightly lower potencies were displayed against PDEs I, II, III and V. In contrast, rolipram exhibited selectivity for PDE IV (3 ± 0.5 μm, n = 3). 3 Ibudilast (IC50 = 0.87 ± 0.37 μm, n = 3), like rolipram (IC50 = 0.20 ± 0.04 μm, n = 3), was a more potent inhibitor of membrane‐bound PDE IV from guinea‐pig eosinophils than of partially purified PDE IV from bovine tracheal smooth muscle. The potency of ibudilast increased when the eosinophil enzyme was solubilised with deoxycholate and NaCl (IC50 = 0.11 ± 0.05 μm, n = 3) or exposed to vanadate/glutathione complex (V/GSH) (IC50 = 0.11 ± 0.02 μm, n = 3). The potency of rolipram was also increased by solubilization (IC50 = 0.012 ± 0.003, n = 3) or V/GSH (IC50 = 0.012 ± 0.003, n = 3). 4 In intact eosinophils, ibudilast (0.032 μm − 20 μm) potentiated isoprenaline‐induced cyclic AMP accumulation in a concentration‐dependent manner, being approximately 20 fold less potent than rolipram. Little or no effect on basal cyclic AMP levels was observed with either compound. The cyclic AMP‐dependent protein kinase activity ratio was significantly increased following incubation of eosinophils with either ibudilast (20 μm) or rolipram (20 μm) in the absence or presence of isoprenaline. 5 Leukotriene B4 (300 nm)‐induced thromboxane generation from guinea‐pig eosinophils was inhibited by ibudilast (IC50 = 11.3 ± 3.7 μm, n = 5) and rolipram (IC50 = 0.280 ± 0.067 μm, n = 5) in a concentration‐dependent manner. 6 Ibudilast (10 nm − 1 μm), whilst generally less potent than rolipram (1 nm − 1 μm), produced concentration‐dependent relaxation of spasmogen (methacholine, histamine, LTD4)‐induced tone in the guinea‐pig isolated tracheal strip. Ibudilast was less potent in reversing the methacholine (IC30 = 1.95 ± 0.40 μm, n = 6)‐induced contraction than those of histamine (IC50 = 0.18 ± 0.70 μm, n = 6) or leukotriene D4 (LTD4, IC50 = 0.12 ± 0.05 μm, n = 6). Rolipram also exhibited a similar pattern of activity, although the difference in potency against methacholine (IC50 = 0.1 ± 0.01 μm, n = 6) compared with the other two spasmogens, histamine (IC50 = 0.034 ± 0.017 μm, n = 7) and LTD4 (IC50 = 0.026 ± 0.008 μm, n = 7), was not as great. 7 These results demonstrate that ibudilast, like rolipram, has several biological actions on the eosinophil and airways smooth muscle which may be attributed to inhibition of cyclic AMP PDE. These actions may account, at least in part, for the recently reported anti‐asthma effects of ibudilast.

The effect of cyclic AMP and cyclic GMP phosphodiesterase inhibitors on the superoxide burst of guinea‐pig peritoneal macrophages

1 The cyclic nucleotide phosphodiesterase (PDE) activity of guinea‐pig peritoneal macrophages was partially characterized and the effects of selective and non‐selective inhibitors of adenosine 3′:5′‐cyclic monophosphate (cyclic AMP PDE) and guanosine 3′:5′‐cyclic monophosphate (cyclic GMP PDE) phosphodiesterases on superoxide generation were investigated using peritoneal macrophages from horse‐serum pretreated guinea‐pigs. 2 The non‐selective PDE inhibitor, 3‐isobutyl‐1‐methylxanthine (IBMX) and the PDE I/V selective inhibitor, zaprinast, inhibited spontaneous superoxide generation with IC50s of 30.7 ± 11.3 μm and 145 ± 17 μm respectively (n = 6 and 5). The concentration‐response curves for the PDE IV selective inhibitors rolipram and Ro20–1724 were biphasic; mean maximum inhibitions were 56.9 ± 5.9% and 66.8 ± 10.5% respectively at 300 μm, but in 2 out of 6 (rolipram) and 2 out of 5 (Ro20–1724) experiments inhibition was < 50%. The PDE III inhibitor SK&F 94120 was without effect. Spontaneous superoxide generation was reduced 57 ± 10% by 1 μm prostaglandin E2 (PGE2) and 62.6 ± 3.76% by 1 μm salbutamol. 3 The increase in superoxide generation elicited by FMLP (10−9−10−5 m) was unaffected by any of the PDE inhibitors studied. Inhibition of FMLP‐stimulated superoxide generation by PGE2 was enhanced in the presence of 10 μm IBMX. 4 Macrophages were found to contain a predominantly membrane bound cyclic AMP PDE (90% of total activity) which was unaffected by cyclic GMP or calcium/calmodulin. The cyclic AMP PDE activity in the cytosolic fraction was enhanced in the presence of calcium/calmodulin. Selective inhibitors of PDE IV inhibited the particulate cyclic AMP PDE activity (IC50s rolipram 1.5 ± 0.3 μm, Ro 20–1724 4.1 ± 0.6 μm) as did the non‐selective inhibitor IBMX (IC50 22 ± 8 μm). The macrophage particulate PDE activity was resistant to inhibition by the PDE III inhibitor SK&F 94836 and the PDE I/V inhibitor, zaprinast. The cytosolic calcium/calmodulin stimulated cyclic AMP hydrolytic activity was inhibited by zaprinast (IC50 ‐ calcium/calmodulin 123 ± 39 μm; + calcium/calmodulin IC50 17.7 ± 6.3 μm). 5 The results indicate that guinea‐pig peritoneal macrophages contain a type IV cyclic AMP PDE which is predominantly membrane associated and a predominantly cytosolic calcium/calmodulin stimulated cyclic AMP PDE. Functional studies suggest that both of these PDE activities contribute to cyclic AMP hydrolysis and regulation of superoxide generation in these cells. Inhibition of spontaneous superoxide generation, but not that stimulated by FMLP, suggests that the activity of PDE inhibitors is subject to functional antagonism but that this can be overcome by enhancing cyclic AMP formation.

Phosphodiesterase type IV inhibitors.

Publisher Summary This chapter summarizes the biochemistry, molecular biology, regulation, and pharmacology of phosphodiesterase IV (PDE IV). The structure–activity relationships (SARs) of known groups of compounds against PDE IV are discussed in detail, as are the in vitro and in vivo actions of PDE IV inhibitors, which have led to the current guarded optimism of their therapeutic potential. Finally, the side effects of these compounds, which may have an important impact on the eventual clinical utility of PDE IV inhibitors, are critically assessed. The next few months will reveal whether the current, guarded optimism in the therapeutic potential of PDE IV inhibitors will be fulfilled. Only a limited amount of information on the clinical efficacy of relatively weak PDE IV inhibitors in asthma and skin disorders (psoriasis) has been published. Several companies currently have, or will have, compounds undergoing clinical evaluation in asthma patients. Although a large body of information from acute animal studies suggests that PDE IV inhibitors will be effective in the treatment of asthma, and bronchodilating activity has been demonstrated in the clinic with weak PDE IV inhibitors and mixed PDE III/IV inhibitors, long-term studies with the newer, highly potent compounds will be required to determine whether they suppress the mucosal inflammation associated with this chronic disease. This will ultimately determine whether they will assume a position of importance in prophylactic asthma therapy.