Methyl xanthine phosphodiesterase inhibitors behave as prostaglandin antagonists in a perfused rat mesenteric artery preparation.

The methyl xanthines, theophylline, caffeine and 3-isobutyl-1 methyl xanthine (MIX) inhibited the pressure responses to noradrnealine, angiotensin II and potassium ions in the isolated perfused mesenteric vascular bed of the male rat. The ID50s for inhibition of responses to noradrenaline were 1.85 mug/ml (0.83 x 10(-5) M) for MIX, 18 mug/ml (1 x 10(-4)M) for theophylline and 133 mug/ml (6.8 x 10(-4) M) for caffeine. Similar ID50 concentrations were found for responses to angiotensin II and potassium. We have previously found that substances which inhibit the three pressor agents equally may be prostaglandin (PG) synthesis inhibitors or PG antagonists. Xanthine itself, cyclic AMP and dibutyrl cyclic AMP had no inhibitory effects on the preparation up to concentrations of 10-2 M. Partial inhibition of PG synthesis by indomethacin shifted the % inhibition/log concentration curve to the left, while addition of exogenous PGE2 shifted it to the right. In preparations completely inhibited by sufficient indomethacin added to the perfusate to block PG synthesis, and then restored by adding 1 or 5 ng/ml PGE2 in addition to the indomethacin, the methyl xanthines again inhibited responses suggesting that they were PG antagonists rather than inhibitors of synthesis or release. In preliminary experiments MIX also inhibited effects of PGF2alpha on rat uterus and PGE1 on guinea pig ileum. Effective concentrations of theophylline were similar to the therapeutic levels in human plasma. PG antagonists may be a major action of methyl xanthines requiring reinterpretation of many experiments which have attributed their effects to PDE inhibition. PGs may also be involved in regulating PDE action.     

Methyl xanthine phosphodiesterase inhibitors behave as prostaglandin antagonists in a perfused rat mesenteric artery preparation

The methyl xanthines, theophylline, caffeine and 3-isobutyl-1 methyl xanthine (MIX) inhibited the pressure responses to noradrenaline, angiotensin II and potassium ions in the isolated perfused mesenteric vascular bed of the male rat. The ID50s for inhibition of responses to noradrenaline were 1.85 μg/ml (0.83 × 10⁻⁵M) for MIX, 18 μg/ml (1 × 10⁻⁴M) for theophylline and 133 μg/ml (6.8 × 10⁻⁴M) for caffeine. Similar ID50 concentrations were found for responses to angiotensin II and potassium. We have previously found that substances which inhibit the three pressor agents equally may be prostaglandin (PG) synthesis inhibitors or PG antagonists. Xanthine itself, cyclic AMP and dibutyryl cyclic AMP had no inhibitory effects on the preparation up to concentrations of 10⁻²M. Partial inhibition of PG synthesis by indomethacin shifted the % inhibition/log concentration curve to the left, while addition of exogeneous PGE2 shifted it to the right. In preparations completely inhibited by sufficient indomethacin added to the perfusate to block PG synthesis, and then restored by adding 1 or 5 ng/ml PGE2 in addition to the indomethacin, the methyl xanthines again inhibited responses suggesting that they were PG antagonists rather than inhibitors of synthesis or release. In preliminary experiments MIX also inhibited effects of PGF2α on rat uterus and PGE1 on guinea pig ileum. Effective concentrations of theophylline were similar to the therapeutic levels in human plasma. PG antagonism may be a major action of methyl xanthines requiring reinterpretation of many experiments which have attributed their effects to PDE inhibition. PGs may also be involved in regulating PDE action.     

Reverse transformation of Chinese hamster ovary cells by methyl xanthines : Structure-function relationships

Using a number of drugs that increase cellular cAMP levels, alterations in the amount of cell surface fibronectin and other transformation parameters were studied in Chinese hamster ovary (CHO) cells. The drugs include db-cAMP, different methylxanthines (theophylline, aminophylline, methyl isobutyl xanthine (MIX), caffeine and theobromine), papaverine and cholera toxin. Methylxanthines that have a methyl group at the seventh position lack reverse transforming potential; those that lack a methyl group at the seventh position induced reverse transformation in CHO cells, causing an increase in surface fibronectin, cell substratum adhesive strength and anchorage dependence for growth. Further, as methyl xanthines are substituted in other positions different from the seventh position, the more efficient they become in restoring normal phenotypic properties; the later agents also induced low saturation density via a cytostatic state causing accumulation of cells in the S and G2 phases of the cycle in contrast to the G1 arrest of normal cells at low saturation density. db-cAMP and cholera toxin induced cell elongation but like caffeine and theobromine, did not induce surface fibronectin. The non-methylxanthine phosphodiesterase inhibitor papaverine induced neither cell elongation nor surface fibronectin but produced a cytostatic effect similar to aminophylline and MIX. These studies suggest that the reverse transformation properties fall into two groups: (a) Differentiation-related properties including cell morphology, parallel alignment and surface matrix fibronectin, etc.; (b) cell cycle-related properties-low saturation density, cell arrest at G1 phase and anchorage-dependent growth. Phosphodiesterase inhibitors reversibly eliminate indefinite division potential of CHO cells by inducing a cytostatic situation and not by inducing a G1-specific arrest.     

Effects of methylxanthines on gonadotropin-induced steroidogenesis and protein synthesis in isolated testis interstitial cells.

The effects of 1-methyl 3-isobutyl xanthine (MIX) and theophylline on basal and gonadotropin-stimulated production of cyclic AMP and testosterone were evaluated in enzyme-dispersed testicular interstitial cells. The actions of these compounds upon precursor incorporation into RNA and protein were also examined in the same cell preparation. The considerable higher potency of MIX as a phosphodiesterase inhibitor was accompanied by a steeper dose-response curve for cyclic AMP recovery in incubation media of hormone-treated cells. Both inhibitors caused increases in basal and hormone-stimulated cyclic AMP levels. Although low concentrations of theophylline and MIX had no effect on the maximum levels of hCG-stimulated testosterone production, 10 mM theophylline and 1 mM MIX significantly inhibited steroidogenesis. Conversely, basal testosterone levels were increased by MIX and theophylline. The higher concentrations of MIX and theophylline also significantly inhibited precursor incorporation into RNA and protein. These actions of phosphodiesterase inhibitors upon RNA and protein synthesis could contribute to their inhibitory effects at high concentrations upon gonadotropin-induced steroidogenesis.     

Agonistic effects of xanthines on tetrodotoxin-sensitive sodium channels in the rat dorsal root ganglion neurons

The effects of four xanthine derivatives, caffeine, caffeine benzoate, theophylline, and bromtheophylline, on sodium channels in internally perfused rat dorsal root ganglion neurons were studied under voltage-clamp and whole-cell patch-clamp conditions. Reversible acceleration, enhancement of the amplitude of sodium currents, and shifts of the current-voltage relation (plotted for their maxima), as well as of the steady-state inactivation curve toward more negative potentials, were observed at external applications of the above substances in the concentrations of 0.2–4.0 mM. Under long exposures, inactivation of sodium currents became slower in a part of the cells. Yet, when the exposure to 4 mM or higher concentrations was longer than 10 min, a rise in the passive conductance was obvious, and functional state of the cells became worse. Blocking effects of the xanthine derivatives on transient or delayed potassium currents were not observed. Thus, agonistic action of xanthines on sodium channels has been demonstrated, and it is supposed that a considerable component of their pharmacological effects is provided by the action on Na⁺/Ca²⁺ exchange.     

Autoradiographical Studies of in Vitro and Chronic in Vivo Effects of Propentofylline on Adenosine A1 and A2 Receptors and NBMPR-Sensitive Nucleoside Transporters

Abstract

Propentofylline is a novel xanthine that has been shown to limit the extent neuronal damage induced by cerebral ischemia in gerbils (DeLeo et al., 1987). This is in contrast to other xanthines, including, caffeine and theophylline, that increase the extent of damage (Rudolphi et al., 1987; Dux et al., 1987). Propentofylline has been demonstrated to decrease adenosine uptake into human erythrocytes (Fredholm and Lindström, 1986), and to increase extracellular concentration of adenosine in ischemic barain (Andine et al., 1990). Therefore, it was proposed that this compound provides protection in cerebral ischemia, in part, by adenosine receptor stimulation due to increased endogenous adenosine levels.     

Caffeine and theophylline analogues: correlation of behavioral effects with activity as adenosine receptor antagonists and as phosphodiesterase inhibitors

The behavioral stimulant effects of xanthines, such as caffeine and theophylline, appear to involve blockade of central adenosine receptors. However, 3-isobutyl-1-methylxanthine (IBMX), a potent phosphodiesterase (PDE) inhibitor, produces behavioral depression. The effects of caffeine analogs on open field behavior of mice and potencies as antagonists of adenosine receptors and as inhibitors of three classes of brain PDE have been compared. 1,7-Dimethyl-3-propargylxanthine, 1,3,7-tripropargylxanthine, and 3,7-dimethyl-1-propargylxanthine, which have high affinity for adenosine receptors and weaker activity as PDE inhibitors, all increase behavioral activity. In contrast, 1,3,7-tripropylxanthine, a more potent inhibitor of the brain calcium-independent (Ca-indep) PDEs than 1,3,7-tripropargylxanthine, produces behavioral depression, even though both analogues are potent adenosine receptor antagonists. 7-Benzyl-IBMX, an active receptor antagonist and selective inhibitor of a brain calcium-dependent (Ca-dep) PDE, produces a slight behavioral activation. Xanthines that are potent adenosine receptor antagonists and relatively weak inhibitors of the Ca-indep PDEs reverse the depressant effects of N6-cyclohexyladenosine, while xanthines, such as 1,3,7-tripropylxanthine, that are potent inhibitors of the Ca-indep PDEs, do not. The results suggest that the behavioral effects of xanthines may be determined primarily by relative activity as adenosine receptor antagonists and as inhibitors of brain Ca-indep PDEs.     

Mechanism for the emetic side effect of xanthine bronchodilators.

The usefulness of xanthine bronchodilators in the treatment of asthma is often limited by the side effects of nausea and vomiting. We investigated the mechanism of emesis induced by xanthines, by examining the roles of phosphodiesterase (PDE) inhibition and adenosine antagonism. Theophylline, enprofylline, 8-phenyltheophylline and isobutylmethylxanthine (IBMX), as well as vehicle, were given to ferrets at doses ranging from 0.1 to 150 mg/kg i.p. The potencies of these compounds in producing emetic responses were ranked IBMX greater than enprofylline greater than theophylline greater than 8-phenyltheophylline. These results correlate well with the relative potencies of the compounds as nonselective PDE inhibitors but do not correlate with their relative potencies as adenosine A1 or A2 receptor antagonists. The emetic responses also correlate well with the previously reported potencies of these xanthines as bronchodilators in guinea pigs. We conclude that the emetic side effect of xanthine bronchodilators results from the inhibition of one or more forms of PDE rather than from adenosine antagonism.     

Differentiation between bronchodilation and universal adenosine antagonism among xanthine derivatives

Relaxant effects and adenosine-antagonism of 3-propyl-xanthine (enprofylline) and 10 different methyl-xanthines were examined in isolated guinea-pig tracheas. The chemical structural requirements for tracheal relaxation were found to be different from those for adenosine antagonism by the xanthine derivatives. All xanthines produced relaxation: Enprofylline was about 5 times more potent than theophylline. However, only xanthines with a methyl in the 1-position consistently antagonized the relaxant effect of adenosine. --Theophylline over a wide range of concentrations (30-900 microM) produced a concentration dependent and surmountable antagonism at nervous adenosine receptors (isolated guinea-pig myenteric-plexus preparations). The same concentrations of enprofylline were almost devoid of antagonism at these adenosine receptors. In mice theophylline (6-24 mg/kg given intraperitoneally) dose-dependently increased locomotor activity while enprofylline (2-48 mg/Kg) was without effect on behaviour. "Non-blocking" xanthines such as enprofylline may be potent bronchodilators but lack many theophylline-like actions. We, therefore, forward the hypothesis that universal adenosine antagonism is both unnecessary and undesirable with xanthine antiasthmatics.     

Potentiating effects of methylxanthines on teratogenicity of mitomycin C in mice

A previous study demonstrated that caffeine strongly potentiated the teratogenic action of mitomycin C in mice. In the present study the effect of methylxanthines including caffeine, theophylline, theobromine (theobromine sodium salicylate), paraxanthine, and 1-methylxanthine was compared in order to analyze the structure-activity relationship. Jcl:ICR mice were injected IP with 3 mg/kg of mitomycin C, immediately followed by SC injection of each methylxanthine on day 11 of gestation. The doses of methylxanthines were calculated so that the mice received 50 mg/kg of caffeine or the equimolecular amount of the other methylxanthines. Fetuses were examined for external malformations on day 18 of gestation. Mitomycin C at 3 mg/kg and the methylxanthines at the doses used were not teratogenic. Combined administration of caffeine or theophylline with mitomycin C produced more than 80% of malformed fetuses. Although less effective than caffeine or theophylline, paraxanthine also significantly increased the incidence of malformed fetuses. Theobromine and 1-methylxanthine were virtually ineffective. From these findings, it is suggested that the methyl group at N-1 position of the xanthines is important for the enhancement but the N-1 methylation alone is ineffective unless accompanied with the substitution of the methyl moiety at the other position(s).     

Modification of radiation-induced division delay by caffeine analogues and dibutyryl cyclic AMP

The mitotic selection procedure for cell cycle analysis was utilized to investigate the concentration-dependent modification of radiation-induced division delay in Chinese hamster ovary (CHO) cells by methyl xanthines (caffeine, theophylline, and theobromine) and by dibutyryl cyclic AMP. The methyl xanthines (concentrations from 0.5 to 1000 micrograms/ml) all reduced radiation-induced division delay with the effect being linear between approximately 100 and 1000 micrograms/ml. After doses of 100-300 rad, delay was reduced by 75, 94 or 83 per cent at 1000 micrograms/ml for each drug, respectively. However, the addition of dibutyryl cyclic AMP had an opposite effect: radiation-induced delay was increased by the concentration range of 0.3 to 300 micrograms/ml. These results indicate that in mammalian cells the control of cell cycle progression and the modification of radiation-induced division delay are not simply related to intracellular levels of cyclic AMP. Rather, there appear to be at least two competing mechanisms which are differentially affected by caffeine analogues or by direct addition of dibutyryl cyclic AMP. The direct effect of caffeine and the methyl xanthines on membrane calcium permeability is considered.     

Chronic Effects of Xanthines on Levels of Central Receptors in Mice

1.Chronic ingestion of caffeine causes a significant increase in levels of A₁-adenosine, nicotinic and muscarinic receptors, serotonergic receptors, GABA_A receptors and L-type calcium channels in cerebral cortical membranes from mice NIH Swiss strain mice.2.Chronic theophylline and paraxanthine had effects similar to those of caffeine except that levels of L-type channels were unchanged. Chronic theobromine, a weak adenosine antagonist, and 1-isobutyl-3-methylxanthine (IBMX), a potent adenosine antagonist and phosphodiesterase inhibitor, caused only an increase in levels of A₁-adenosine receptors. A combination of chronic caffeine and IBMX had the same effects on receptors as caffeine alone. Chronic 3,7-dimethyl-1-propargylxanthine (DMPX), a somewhat selective A_2A-antagonist, caused only an increase in levels of A₁-adenosine receptors. Pentoxyfylline, an adenosine-uptake inhibitor inactive at adenosine receptors, had no effect on receptor levels or calcium channels.3.A comparison of plasma and brain levels of xanthines indicated that caffeine penetrated more readily and attained somewhat higher brain levels than theophylline or theobromine. Penetration and levels were even lower for IBMX, paraxanthine, DMPX, and pentoxyfylline.4.The results suggest that effective blockade of both A₁ and A_2A-adenosine receptors is necessary for the full spectrum of biochemical changes elicited by chronic ingestion of xanthines, such as caffeine, theophylline, and paraxanthine.     

Catabolism of caffeine and related purine alkaloids in leaves ofCoffea arabica L.

In a study of purine alkaloid catabolism pathways in coffee,¹⁴C-labelled theobromine, caffeine, theophylline and xanthine were incubated with leaves ofCoffea arabica. Incorporation of label into¹⁴CO₂ was determined and methanol-soluble metabolites were analysed by high-performance liquid chromatography-radiocounting. The data obtained demonstrate catabolism of caffeine theophylline 3-methylxanthine xanthine. Xanthine is degraded further by the conventional purine catabolism pathway to CO₂ and NH₃ via uric acid, allantoin and allantoic acid. The conversion of caffeine to theophylline is the rate-limiting step in purine alkaloid catabolism and provides a ready explanation for the high concentration of endogenous caffeine found inC. arabica leaves. Although theobromine is converted primarily to caffeine, a small portion of the theobromine pool appears to be degraded to xanthine by a caffeine-independent pathway. In addition to being broken down to CO₂, via the purine catabolism pathway, xanthine is metabolised to 7-methylxanthine. Metabolism of [2-¹⁴C]xanthine byC. arabica leaves in the presence of 5 mM allopurinol results in very large increases in incorporation of radioactivity into 7-methylxanthine as degradation of the substrate via the purine catabolism pathway is blocked. The identity of 7-methylxanthine in these studies was confirmed by gas chromatography-mass spectrometry analysis.Abbreviations HPLC-RC high-performance liquid chromatography-radiocounting This work was supported by the British Council which provided H.A. with Japan-UK travel grants. F.M.G. was supported by a Biotechnology and Biological Sciences Research Council grant to A.C.     

Degradation of caffeine and related methylxanthines bySerratia marcescens isolated from soil under coffee cultivation

A strain of Serratia marcescens showing the ability to degrade caffeine and other methylxanthines was isolated from soil under coffee cultivation. Growth was observed only with xanthines methylated at the 7 position (caffeine, 1,3,7-dimethylxanthine; paraxanthine, 1,7-dimethylxanthine; theobromine, 3,7-dimethylxanthine and 7-methylxanthine). Paraxanthine and theobromine were released in liquid medium when caffeine was used as the sole source of carbon and nitrogen. When paraxanthine or theobromine were used, 3-methylxanthine, 7-methylxanthine, and xanthine were detected in the liquid medium. Serratia marcescens did not grow with theophylline (1,3-dimethylxanthine), 1-methylxanthine, and 3-methylxanthine, and poor growth was observed with xanthine. Methyluric acid formation from methylxanthines was tested in cell-free extracts by measuring dehydrogenase reduction of tetrazolium salt in native-polyacrylamide gel electrophoresis gel. Activity was observed for all methylxanthines, even those with which no bacterial growth was observed. Our results suggest that in this strain of S. marcescens caffeine is degraded to theobromine (3,7-dimethylxanthine) and/or paraxanthine (1,7-dimethylxanthine), and subsequently to 7-methylxanthine and xanthine. Methyluric acid formation could not be confirmed. Correspondence to: Paulo Mazzafera.     

Effects of enprofylline and theophylline may show the role of adenosine

It is well established that at low and clinically relevant concentrations theophylline (and caffeine) exerts antagonism at cell surface receptor sites for adenosine. However, it is not known which actions of theophylline are due to adenosine antagonism, because theophylline apparently activates other cellular mechanisms at the same low concentrations. Investigations into the actions of xanthines and their structure activity relationships have identified xanthine compounds like enprofylline (3-propylxanthine) that only has some actions in common with theophylline and that has a negligible ability to antagonize adenosine. Enprofylline is a more potent smooth muscle relaxant and antiasthmatic drug than theophylline but does not produce, e.g., theophylline-like diuretic effects, CNS-stimulant behavioural effects (restlessness - seizures), gastric secretory effects and release of free fatty acids. It is proposed that pharmacodynamic dissimilarities between enprofylline and theophylline may indicate physiological roles of adenosine.     

Interaction of methyl-xanthines with myeloperoxidase. An anti-inflammatory mechanism.

1. Inhibition of myeloperoxidase (MPO)-catalyzed reactions by methyl-substituted xanthines has been investigated. 2. Except for theobromine and caffeine, all xanthines tested were potent inhibitors of the MPO-H2O2-Cl- system. 3. In contrast to methyl substitution in the 1 or 8 position of xanthine, substitution in the 3 or 7 position had a marked effect on the inhibition of MPO catalysis. 4. Two different inhibitory mechanisms were induced; scavenging of hypochlorous acid (HOCl) generated by the MPO system and accumulation of Compound II (ferryl MPO) which is inactive as a catalyst of Cl- oxidation.     

De-intercalation of ethidium bromide and acridine orange by xanthine derivatives and their modulatory effect on anticancer agents: a study of DNA-directed toxicity enlightened by time correlated single photon counting

Time Correlated Single Photon Counting (TCSPC) was used for the first time to analyze the effect/changes in the mode of intercalation of ethidium bromide (EtBr) and acridine orange (AO) to calf thymus DNA brought about due to interaction of naturally occurring methylxanthines such as theophylline (X1), theobromine (X2) and caffeine (X3). UV absorption and fluorescence studies were also carried to observe the behaviour of these xanthines on the modulation of the binding mode of anticancer agents (cisplatin, novantrone, and actinomycin D) and certain intercalating dyes (EtBr and AO) to DNA. In TCSPC analysis we found that when the concentration of the drugs (X1, X2 and X3) increased from 0.025 mM to 2 mM i.e. P/D 2.4 to P/D 0.03 reduction in intercalation of EtBr and AO was observed, suggesting that xanthine derivatives could play very important role in reducing the DNA-directed toxicity in a dose dependent manner. In TCSPC, the amplitude of smaller lifetime component A(1) and higher lifetime component A(2) are attributed to free and intercalated dye concentration and their variation could indicate the process of intercalation or reduced intercalation of EtBr and AO by xanthine derivatives. We found that at the maximum drug concentration the smaller lifetime component A(1) was increased by 7-8% and 17-37% in EtBr and AO intercalated complex respectively. Also the changes in lifetime and fluorescence decay profile were observed for the DNA-intercalated dyes before and after treatment with xanthines. Especially, at maximum P/D 0.03 the lifetime of DNA-intercalated EtBr and AO reduced by 1-2 ns. The present analysis reveals that xanthines are able to interact with free dyes and also with intercalated dyes, suggesting that when they interact with free dyes they might inhibit the further intercalation of dye molecules to DNA and the interaction with intercalated dyes might lead to displacement of the dyes resulting in de-intercalation. The results obtained from UV and fluorescence spectroscopy also support the present investigation of probable interaction of xanthines with the DNA damaging agents in modulating/reducing the DNA-directed toxicity.     

The inhibitory effect of xanthine derivatives on alkaline phosphatase in the rat brain

Summary Histochemical and biochemical studies were carried out on the inhibition of alkaline phosphatase (Al-P) activity in rat cerebral cortex with various methylxanthine derivatives.The histochemical study revealed that Al-P activity was completely inhibited with 2 mM theophylline or aminophylline, only slightly inhibited with 5 mM of xanthine, and no way inhibited even with 5 mM of diprophylline or caffeine.The biochemical study showed that Al-P activity was markedly inhibited by 1 mM theophylline, to 36% of the control value, and equally markedly by 1 mM aminophylline to 26% of the control value. It was only inhibited to 99% of the control value even by 5 mM diprophylline and conversely slightly activated, to 110% of the control value, by caffeine.The relationship between the pharmacological activities of methylxanthine derivatives and Al-P was studied, and the biological role of Al-P in the central nervous system was also discussed.     

The adenosine A2A antagonistic properties of selected C8-substituted xanthines

The adenosine A_2A receptor is considered to be an important target for the development of new therapies for Parkinson’s disease. Several antagonists of the A_2A receptor have entered clinical trials for this purpose and many research groups have initiated programs to develop A_2A receptor antagonists. Most A_2A receptor antagonists belong to two different chemical classes, the xanthine derivatives and the amino-substituted heterocyclic compounds. In an attempt to discover high affinity A_2A receptor antagonists and to further explore the structure–activity relationships (SARs) of A_2A antagonism by the xanthine class of compounds, this study examines the A_2A antagonistic properties of series of (E)-8-styrylxanthines, 8-(phenoxymethyl)xanthines and 8-(3-phenylpropyl)xanthines. The results document that among these series, the (E)-8-styrylxanthines have the highest binding affinities with the most potent homologue, (E)-1,3-diethyl-7-methyl-8-[(3-trifluoromethyl)styryl]xanthine, exhibiting a K_i value of 11.9 nM. This compound was also effective in reversing haloperidol-induced catalepsy in rats, providing evidence that it is in fact an A_2A receptor antagonist. The importance of substitution at C8 with the styryl moiety was demonstrated by the finding that none of the 8-(phenoxymethyl)xanthines and 8-(3-phenylpropyl)xanthines exhibited high binding affinities for the A_2A receptor.     

Induction of alkaline phosphatase activity in HeLa cells by sodium butyrate

Summary The induction of HeLa cell alkaline phosphatase activity by sodium butyrate could be inhibited by the coadministration of caffeine or theophylline. The inhibitions were dose dependent, and at any given concentration the potency was theophylline > caffeine. Although the induction by sodium butyrate was more sensitive to the inhibition by the xanthines than was that produced by 5-iodo-2′-deoxyuridine, the magnitudes of the increases in cyclic AMP concentrations after treatment with the xanthines were similar in the inhibition of both types of induction. The induction of alkaline phosphatase activity by sodium butyrate also produced a shift in the thermostability pattern of the enzyme, with a proportionately greater increase in the heat-labile, rather than heat-stable, from of the activity. Supported by National Cancer Institute Grant CA16460.