PRECAUTIONS General Careful consideration of the various interacting drugs and physiologic conditions that can alter theophylline clearance and require dosage adjustment should occur prior to initiation of theophylline therapy and prior to increases in theophylline dose (see WARNINGS ). Monitoring Serum Theophylline Concentrations Serum theophylline concentration measurements are readily available and should be used to determine whether the dosage is appropriate. Specifically, the serum theophylline concentration should be measured as follows: Before making a dose increase to determine whether the serum concentration is sub-therapeutic in a patient who continues to be symptomatic. Whenever signs or symptoms of theophylline toxicity are present. Whenever there is a new illness, worsening of an existing concurrent illness or a change in the patient's treatment regimen that may alter theophylline clearance (e.g., fever greater than 102°F sustained for greater than or equal to 24 hours, hepatitis, or drugs listed in Table II are added or discontinued). In patients who have received no theophylline in the previous 24 hours, a serum concentration should be measured 30 minutes after completion of the intravenous loading dose to determine whether the serum concentration is less than 10 mcg/mL indicating the need for an additional loading dose or greater than 20 mcg/mL indicating the need to delay starting the constant IV infusion. Once the infusion has begun, a second measurement should be obtained after one expected half life (e.g., approximately 4 hours in children age 1 to 9 years and 8 hours in non-smoking adults; see Table I for the expected half life in additional patient populations). The second measurement should be compared to the first to determine the direction in which the serum concentration has changed. The infusion rate can then be adjusted before steady state is reached in an attempt to prevent an excessive or sub-therapeutic theophylline concentration from being achieved. If a patient has received theophylline in the previous 24 hours, the serum concentration should be measured before administering an intravenous loading dose to make sure that it is safe to do so. If a loading dose is not indicated (i.e., the serum theophylline concentration is greater than or equal to 10 mcg/mL), a second measurement should be obtained as above at the appropriate time after starting the intravenous infusion. If, on the other hand, a loading dose is indicated (see DOSAGE AND ADMINISTRATION for guidance on selection of the appropriate loading dose), a second blood sample should be obtained after the loading dose and a third sample should be obtained one expected half-life after starting the constant infusion to determine the direction in which the serum concentration has changed. Once the above procedures related to initiation of intravenous theophylline infusion have been completed, subsequent serum samples for determination of theophylline concentration should be obtained at 24-hour intervals for the duration of the infusion. The theophylline infusion rate should be increased or decreased as appropriate based on the serum theophylline levels. When signs or symptoms of theophylline toxicity are present, the intravenous infusion should be stopped and a serum sample for theophylline concentration should be obtained as soon as possible, analyzed immediately, and the result reported to the clinician without delay. In patients in whom decreased serum protein binding is suspected (e.g., cirrhosis, women during the third trimester of pregnancy), the concentration of unbound theophylline should be measured and the dosage adjusted to achieve an unbound concentration of 6–12 mcg/mL. Saliva concentrations of theophylline cannot be used reliably to adjust dosage without special techniques. Clinical evaluation and periodic laboratory determinations are necessary to monitor changes in fluid balance, electrolyte concentrations, and acid-base balance during prolonged therapy or whenever the condition of the patient warrants such evaluation. Do not use plastic containers in series connection. If administration is controlled by a pumping device, care must be taken to discontinue pumping action before the container runs dry or air embolism may result. If administration is not controlled by a pumping device, refrain from applying excessive pressure (greater than 300mmHg) causing distortion to the container such as wringing or twisting. Such handling could result in breakage of the container. This solution is intended for intravenous administration using sterile equipment. It is recommended that intravenous administration apparatus be replaced at least once every 24 hours. Use only if solution is clear and container and seals are intact. Effects on Laboratory Tests As a result of its pharmacological effects, theophylline at serum concentrations within the 10–20 mcg/mL range modestly increases plasma glucose (from a mean of 88 mg% to 98 mg%), uric acid (from a mean of 4 mg/dl to 6 mg/dl), free fatty acids (from a mean of 451 µEq/L to 800 µEq/L, total cholesterol (from a mean of 140 vs 160 mg/dl), HDL (from a mean of 36 to 50 mg/dl), HDL/LDL ratio (from a mean of 0.5 to 0.7), and urinary free cortisol excretion (from a mean of 44 to 63 mcg/24 hr). Theophylline at serum concentrations within the 10–20 mcg/mL range may also transiently decrease serum concentrations of triiodothyronine (144 before, 131 after one week and 142 ng/dl after 4 weeks of theophylline). The clinical importance of these changes should be weighed against the potential therapeutic benefit of theophylline in individual patients. Drug Interactions Theophylline interacts with a wide variety of drugs. The interaction may be pharmacodynamic, i.e., alterations in the therapeutic response to theophylline or another drug or occurrence of adverse effects without a change in serum theophylline concentration. More frequently, however, the interaction is pharmacokinetic, i.e., the rate of theophylline clearance is altered by another drug resulting in increased or decreased serum theophylline concentrations. Theophylline only rarely alters the pharmacokinetics of other drugs. The drugs listed in Table II have the potential to produce clinically significant pharmacodynamic or pharmacokinetic interactions with theophylline. The information in the "Effect" column of Table II assumes that the interacting drug is being added to a steady-state theophylline regimen. If theophylline is being initiated in a patient who is already taking a drug that inhibits theophylline clearance (e.g., cimetidine, erythromycin), the dose of theophylline required to achieve a therapeutic serum theophylline concentration will be smaller. Conversely, if theophylline is being initiated in a patient who is already taking a drug that enhances theophylline clearance (e.g., rifampin), the dose of theophylline required to achieve a therapeutic serum theophylline concentration will be larger. Discontinuation of a concomitant drug that increases theophylline clearance will result in accumulation of theophylline to potentially toxic levels, unless the theophylline dose is appropriately reduced. Discontinuation of a concomitant drug that inhibits theophylline clearance will result in decreased serum theophylline concentrations, unless the theophylline dose is appropriately increased. The drugs listed in Table III have either been documented not to interact with theophylline or do not produce a clinically significant interaction (i.e., less than 15% change in theophylline clearance). The listing of drugs in Tables II and III are current as of September 1, 1995. New interactions are continuously being reported for theophylline, especially with new chemical entities. The clinician should not assume that a drug does not interact with theophylline if it is not listed in Table II. Before addition of a newly available drug in a patient receiving theophylline, the package insert of the new drug and/or the medical literature should be consulted to determine if an interaction between the new drug and theophylline has been reported. Table II. Clinically significant drug interactions with theophylline.Refer to PRECAUTIONS, Drug Interactions for further information regarding table. Drug Type of Interaction EffectAverage effect on steady state theophylline concentration or other clinical effect for pharmacologic interactions. Individual patients may experience larger changes in serum theophylline concentration than the value listed. Adenosine Theophylline blocks adenosine receptors. Higher doses of adenosine may be required to achieve desired effect. Alcohol A single large dose of alcohol (3 mL/kg of whiskey) decreases theophylline clearance for up to 24 hours. 30% increase Allopurinol Decreases theophylline clearance at allopurinol doses greater than or equal to 600 mg/day. 25% increase Aminoglutethimide Increases theophylline clearance by induction of microsomal enzyme activity. 25% decrease Carbamazepine Similar to aminoglutethimide. 30% decrease Cimetidine Decreases theophylline clearance by inhibiting cytochrome P450 1A2. 70% increase Ciprofloxacin Similar to cimetidine. 40% increase Clarithromycin Similar to erythromycin. 25% increase Diazepam Benzodiazepines increase CNS concentrations of adenosine, a potent CNS depressant, while theophylline blocks adenosine receptors. Larger diazepam doses may be required to produce desired level of sedation. Discontinuation of theophylline without reduction of diazepam dose may result in respiratory depression. Disulfiram Decreases theophylline clearance by inhibiting hydroxylation and demethylation. 50% increase Enoxacin Similar to cimetidine. 300% increase Ephedrine Synergistic CNS effects. Increased frequency of nausea, nervousness, and insomnia. Erythromycin Erythromycin metabolite decreases theophylline clearance by inhibiting cytochrome P450 3A3. 35% increase. Erythromycin steady-state serum concentrations decrease by a similar amount. Estrogen Estrogen containing oral contraceptives decrease theophylline clearance in a dose-dependent fashion. The effect of progesterone on theophylline clearance is unknown. 30% increase Flurazepam Similar to diazepam. Similar to diazepam. Fluvoxamine Similar to cimetidine. Similar to cimetidine. Halothane Halothane sensitizes the myocardium to catecholamines, theophylline increases release of endogenous catecholamines. Increased risk of ventricular arrhythmias. Interferon, human recombinant alpha-A Decreases theophylline clearance. 100% increase Isoproterenol (IV) Increases theophylline clearance. 20% decrease Ketamine Pharmacologic May lower theophylline seizure threshold. Lithium Theophylline increases renal lithium clearance. Lithium dose required to achieve a therapeutic serum concentration increased an average of 60%. Lorazepam Similar to diazepam. Similar to diazepam. Methotrexate (MTX) Decreases theophylline clearance. 20% increase after low dose MTX, higher dose MTX may have a greater effect. Mexiletine Similar to disulfiram. 80% increase Midazolam Similar to diazepam. Similar to diazepam. Moricizine Increases theophylline clearance. 25% decrease Pancuronium Theophylline may antagonize non-depolarizing neuromuscular blocking effects; possibly due to phosphodiesterase inhibition. Larger dose of pancuronium may be required to achieve neuromuscular blockade. Pentoxifylline Decreases theophylline clearance. 30% increase Phenobarbital (PB) Similar to aminoglutethimide. 25% decrease after two weeks of concurrent PB. Phenytoin Phenytoin increases theophylline clearance by increasing microsomal enzyme activity. Theophylline decreases phenytoin absorption. Serum theophylline and phenytoin concentrations decrease about 40%. Propafenone Decreases theophylline clearance and pharmacologic interaction. 40% increase. Beta-2 blocking effect may decrease efficacy of theophylline. Propranolol Similar to cimetidine and pharmacologic interaction. 100% increase. Beta-2 blocking effect may decrease efficacy of theophylline. Rifampin Increases theophylline clearance by increasing cytochrome P450 1A2 and 3A3 activity. 20–40% decrease Sulfinpyrazone Increases theophylline clearance by increasing demethylation and hydroxylation. Decreases renal clearance of theophylline. 20% decrease Tacrine Similar to cimetidine, also increases renal clearance theophylline. 90% increase Thiabendazole Decreases theophylline clearance. 190% increase Ticlopidine Decreases theophylline clearance. 60% increase Troleandomycin Similar to erythromycin. 33–100% increase depending on troleandomycin dose. Verapamil Similar to disulfiram. 20% increase Table III. Drugs that have been documented not to interact with theophylline or drugs that produce no clinically significant interaction with theophylline.Refer to PRECAUTIONS, Drug Interactions for information regarding table. albuterol, systemic and inhaled medroxyprogesterone amoxicillin methylprednisolone ampicillin, with or without sulbactam metronidazole atenolol metoprolol azithromycin nadolol caffeine, dietary ingestion nifedipine cefaclor nizatidine co-trimoxazole norfloxacin (trimethoprim and sulfamethoxazole) ofloxacin diltiazem omeprazole dirithromycin prednisone, prednisolone enflurane ranitidine famotidine rifabutin felodipine roxithromycin finasteride sorbitol hydrocortisone (purgative doses do not inhibit isoflurane theophylline absorption) isoniazid sucralfate isradipine terbutaline, systemic influenza vaccine terfenadine ketoconazole tetracycline lomefloxacin tocainide mebendazole The Effect of Other Drugs on Theophylline Serum Concentration Measurements Most serum theophylline assays in clinical use are immunoassays which are specific for theophylline. Other xanthines such as caffeine, dyphylline, and pentoxifylline are not detected by these assays. Some drugs (e.g., cefazolin, cephalothin), however, may interfere with certain HPLC techniques. Caffeine and xanthine metabolites in neonates or patients with renal dysfunction may cause the reading from some dry reagent office methods to be higher than the actual serum theophylline concentration. Carcinogenesis, Mutagenesis, and Impairment of Fertility Long term carcinogenicity studies have been carried out in mice (oral doses 30–150 mg/kg) and rats (oral doses 5–75 mg/kg). Results are pending. Theophylline has been studied in Ames salmonella, in vivo and in vitro cytogenetics, micronucleus and Chinese hamster ovary test systems and has not been shown to be genotoxic. In a 14 week continuous breeding study, theophylline, administered to mating pairs of B6C3F1 mice at oral doses of 120, 270 and 500 mg/kg (approximately 1.0–3.0 times the human dose on a mg/m2 basis) impaired fertility, as evidenced by decreases in the number of live pups per litter, decreases in the mean number of litters per fertile pair, and increases in the gestation period at the high dose as well as decreases in the proportion of pups born alive at the mid and high dose. In 13 week toxicity studies, theophylline was administered to F344 rats and B6C3F1 mice at oral doses of 40–300 mg/kg (approximately 2.0 times the human dose on a mg/m2 basis). At the high dose, systemic toxicity was observed in both species including decreases in testicular weight. Pregnancy Category C Teratogenic Effects There are no adequate and well-controlled studies in pregnant women. In animal reproduction studies, maternal doses of theophylline less than one to two times the maximum recommended oral dose in humans caused fetal harm, including fetal malformations. Asthma is a serious and potentially life-threatening condition. Poorly controlled asthma during pregnancy is associated with adverse outcomes for mother and fetus. Theophylline should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Population-based studies and post-marketing adverse event reporting of theophylline use during human pregnancy have not demonstrated an increased risk of major congenital anomalies. However, most studies were not large enough to detect a less than two fold increase in risk for congenital anomalies. Post-marketing data are reported voluntarily and do not always reliably estimate the frequency of particular adverse outcomes. In animal reproduction studies, theophylline produced teratogenic effects when pregnant mice, rats and rabbits were dosed during the period of organogenesis. In mice, a single intraperitoneal dose at and above 100 mg/kg (approximately equal to the maximum recommended oral dose for adults on a mg/m2 basis) produced cleft palate and digital abnormalities. Micromelia, micrognathia, clubfoot, subcutaneous hematoma, open eyelids, and embryolethality were observed at doses approximately 2 times the maximum recommended oral dose for adults on a mg/m2 basis. In rats dosed from conception through organogenesis, an oral dose of 150 mg/kg/day (approximately 2 times the maximum recommended oral dose for adults on a mg/m2 basis) produced digital abnormalities. Embryolethality occurred at a subcutaneous dose of 200 mg/kg/day (approximately 4 times the maximum recommended oral dose for adults on a mg/m2 basis). In rabbits dosed intravenously throughout organogenesis 60 mg/kg/day (approximately 2 times the maximum recommended oral dose for adults on a mg/m2 basis), caused cleft palate and was embryolethal. This dose was maternally toxic as one doe died and clinical signs of toxicity occurred in others. Doses at and above 15 mg/kg/day (less than the maximum recommended oral dose for adults on a mg/m2 basis) increased the incidence of skeletal variations. Nursing Mothers Theophylline is excreted into breast milk and may cause irritability or other signs of mild toxicity in nursing human infants. The concentration of theophylline in breast milk is about equivalent to the maternal serum concentration. An infant ingesting a liter of breast milk containing 10–20 mcg/mL of theophylline per day is likely to receive 10–20 mg of theophylline per day. Serious adverse effects in the infant are unlikely unless the mother has toxic serum theophylline concentrations. Pediatric Use Theophylline is safe and effective for the approved indications in pediatric patients (see INDICATIONS AND USAGE ). The constant infusion rate of intravenous theophylline must be selected with caution in pediatric patients since the rate of theophylline clearance is highly variable across the age range of neonates to adolescents (see CLINICAL PHARMACOLOGY , Table I, WARNINGS , and DOSAGE AND ADMINISTRATION , Table V). Due to the immaturity of theophylline metabolic pathways in pediatric patients under the age of one year, particular attention to dosage selection and frequent monitoring of serum theophylline concentrations are required when theophylline is prescribed to pediatric patients in this age group. Geriatric Use Elderly patients are at significantly greater risk of experiencing serious toxicity from theophylline than younger patients due to pharmacokinetic and pharmacodynamic changes associated with aging. Theophylline clearance is reduced in patients greater than 60 years of age, resulting in increased serum theophylline concentrations in response to a given theophylline infusion rate. Protein binding may be decreased in the elderly resulting in a larger proportion of the total serum theophylline concentration in the pharmacologically active unbound form. Elderly patients also appear to be more sensitive to the toxic effects of theophylline after chronic overdosage than younger patients. For these reasons, the maximum infusion rate of theophylline in patients greater than 60 years of age ordinarily should not exceed 17 mg/hr unless the patient continues to be symptomatic and the steady state serum theophylline concentration is less than 10 mcg/mL (see DOSAGE AND ADMINISTRATION ). Theophylline infusion rate greater than 17 mg/hr should be prescribed with caution in elderly patients.