Plasma Concentrations In Excess Research Articles

Abstract 1176: Preclinical characterization of the pharmacokinetic-pharmacodynamics-efficacy relationship of novel allosteric SHP2 inhibitors

Abstract SHP2 is a non-receptor protein tyrosine phosphatase downstream of receptor tyrosine kinases (RTK). Mutations yielding constitutive activation of SHP2 primarily lead to activation of the MAPK pathway and have been found in multiple tumor types. These observations make SHP2 a potentially promising therapeutic target for the treatment of cancers with RTK dependence. Recently, a novel allosteric mechanism of SHP2 inhibition was identified where the autoinhibited form of SHP2 is stabilized via small molecule binding. Herein we describe efforts to characterize the pharmacokinetic (PK)/pharmacodynamic (PD)/efficacy relationship of orally bioavailable novel allosteric SHP2 inhibitors. Single dose PK/PD studies were conducted in nude mice engrafted with the EGFR amplified esophageal squamous cell carcinoma cell line, KYSE520. The allosteric SHP2 inhibitor SHP099 achieved time and dose dependent increases in plasma concentrations and concomitant reductions in tumor pERK that could be described by an Emax model. Tumor pERK levels were reduced by 50 to 70% when SHP099 unbound plasma concentrations exceeded the in vitro cellular PD IC50, suggesting that exposure above this threshold was required for pathway inhibition in vivo. A second allosteric SHP2 inhibitor, SHP065, was also profiled in vivo and yielded data consistent with this hypothesis. Two additional allosteric SHP2 inhibitors (SHP156, SHP393) that achieved unbound plasma concentrations approximating their cellular IC50s failed to modulate tumor pERK. The totality of these data support the hypothesis that unbound plasma concentrations in excess of the cellular IC50 is required for allosteric SHP2 inhibitors to inhibit the MAPK pathway in vivo. We further demonstrated that SHP099 achieves dose dependent inhibition of KYSE520 tumor xenograft growth in nude mice. Integration of the antitumor efficacy data and pERK inhibition data revealed a direct linear relationship between tumor growth inhibition and the fraction of time between dosing intervals in which pERK is inhibited by at least 50%. To test this model, SHP099 PK data from nude rats was applied to the Emax model and the resulting predicted PD responses were applied to the PD/efficacy model to predict SHP099 anti-tumor efficacy in nude rats. Data generated from a SHP099 KYSE520 efficacy study in nude rats demonstrated that the exposure/response model was remarkably robust. Doses of 8 mg/kg qd, 25 mg/kg qd, or 75 mg/kg q2d yielded observed T/C of 70, 14, and 18%, respectively; versus a model predicted T/C of 75, 6, and 6%, respectively. In summary, we describe for the first time successful efforts to characterize the PK/PD/efficacy relationship of novel allosteric SHP2 inhibitors. These exposure/response models served as a basis for further allosteric SHP2 inhibitor drug discovery efforts and begin to inform rational approaches to dose and schedule selection in clinic. Citation Format: Minying Pu, Laura R. La Bonte, Stan Spence, Kathy Hsiao, Shumei Liu, Brant Firestone, Ping Wang, Pascal D. Fortin, Ying-Nan P. Chen, Matthew J. LaMarche, Matthew J. Meyer. Preclinical characterization of the pharmacokinetic-pharmacodynamics-efficacy relationship of novel allosteric SHP2 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1176. doi:10.1158/1538-7445.AM2017-1176

Abstract A143: A preclinical in vivo model to assess pharmacodynamic inhibition of ACK1.

Abstract Introduction: Activated Cdc42-associated kinase (ACK1, TNK2) has been reported to be over-expressed in a range of tumor types including prostate and lung cancer, and has been suggested as a potential novel anticancer target. A number of kinase inhibitors currently in clinical development show inhibition of ACK1 biochemical activity in enzyme assays, yet their ability to inhibit ACK1 in vivo is unknown. This study describes the development of a pharmacodynamic model to evaluate ACK1 target coverage in vivo. Methods: MDA-MB-231 cells were engineered to over-express ACK1 under the control of a doxycycline-inducible promoter (MDA-MB-231-ACK1). MDA-MB-231-ACK1 cells were grown as subcutaneous xenografts in MF-1 nude mice and ACK1 expression induced by dosing doxycycline by oral gavage. Saracatinib/AZD-0530 (100mg/kg qd) or ASP-3026 (50mg/kg bid) were dosed for four consecutive days to reach steady state exposures at approximately MTD. Pharmacodynamic inhibition of ACK1 autophosphorylation was assessed using an in-house MSD assay and confirmed by Western blotting, in order to evaluate target inhibition in vivo. Results: ASP-3026 and saracatinib showed inhibition of cellular pACK1 of MDA-MB-231-ACK1 cells in vitro, with IC50s of 0.12 μM and 1.1 μM, respectively. Using the same cell line in vivo, expression of total and phosphorylated ACK1 was induced by daily oral dosing of doxycycline. We used this model to evaluate PK-PD correlations of ASP-3026 and saracatinib using pACK1 as a marker of target inhibition. Good plasma and tumor exposure of ASP-3026 was achieved at 50mg/kg bid and no drug-drug interactions were observed in combination with doxycycline. ASP-3026 showed 50-80% inhibition of pACK1 in vivo consistent with unbound plasma concentrations in excess of in vitro IC50 and in good agreement with predictions. Good exposure of saracatinib was observed at 100 mg/kg qd with total plasma Cmax of >4μM in excess of maximally tolerated clinical exposures (0.82μM). Tumor exposures of saracatinib were up to 10 x higher than plasma exposures (Cmax >40 μM), and in considerable excess of the cellular IC50, yet no ACK1 inhibition was observed in vivo. Conclusions: A robust PD model was established to evaluate ACK1 inhibition in vivo. Up to 80% inhibition of ACK1 in vivo was observed with ASP-3026, falling to approximately 50% at Cmin, suggesting that this agent does not completely inhibit ACK1 in vivo at steady state. In contrast, saracatinib did not significantly inhibit ACK1 at any of the exposures achieved, suggesting that it has limited in vivo activity against this target. Unbound plasma exposures predicted in vivo PD responses very well, whereas tumor exposures of these agents were not predictive of target inhibition. This model can be incorporated into a screening cascade for prioritising in vivo active ACK1 inhibitors. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A143. Citation Format: Mark Albertella, Shilina Roman, Mike Briggs, Aaron Cranston, Ralph Graeser, George Hynd, David Jones, Peter Lockey, Akbor Malik, Stephen Price, Susanna Stimpson, Patrizia Tisselli, Janine Arts, Simon Green. A preclinical in vivo model to assess pharmacodynamic inhibition of ACK1. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A143.

MK-0448, a Specific Kv1.5 Inhibitor

Background— We evaluated the viability of I Kur as a target for maintenance of sinus rhythm in patients with a history of atrial fibrillation through the testing of MK-0448, a novel I Kur inhibitor. Methods and Results— In vitro MK-0448 studies demonstrated strong inhibition of I Kur with minimal off-target activity. In vivo MK-0448 studies in normal anesthetized dogs demonstrated significant prolongation of the atrial refractory period compared with vehicle controls without affecting the ventricular refractory period. In studies of a conscious dog heart failure model, sustained atrial fibrillation was terminated with bolus intravenous MK-0448 doses of 0.03 and 0.1 mg/kg. These data led to a 2-part first-in-human study: Part I evaluated safety and pharmacokinetics, and part II was an invasive electrophysiological study in healthy subjects. MK-0448 was well-tolerated with mild adverse experiences, most commonly irritation at the injection site. During the electrophysiological study, ascending doses of MK-0448 were administered, but no increases in atrial or ventricular refractoriness were detected, despite achieving plasma concentrations in excess of 2 μmol/L. Follow-up studies in normal anesthetized dogs designed to assess the influence of autonomic tone demonstrated that prolongation of atrial refractoriness with MK-0448 was markedly attenuated in the presence of vagal nerve simulation, suggesting that the effects of I Kur blockade on atrial repolarization may be negated by enhanced parasympathetic neural tone. Conclusions— The contribution of I Kur to human atrial electrophysiology is less prominent than in preclinical models and therefore is likely to be of limited therapeutic value for the prevention of atrial fibrillation.

CYP2D6 Genotype Information to Guide Pimozide Treatment in Adult and Pediatric Patients

The occurrence of pimozide-induced arrhythmias is concentration dependent. Hence, it is important for prescribers to consider causes of increased pimozide exposure. This article summarizes the U.S. Food and Drug Administration's (FDA's) review of drug interaction and pharmacogenomic studies and discusses pharmacokinetic simulations we performed to develop new cytochrome P450 2D6 (CYP2D6) genotype-guided dosing recommendations for pimozide. Pharmacokinetic parameters by CYP2D6 genotype were derived from a published single-dose pharmacogenomic study of pimozide. We simulated what pimozide exposures would result from a multiple-dose scenario in different CYP2D6 genotype groups: extensive, intermediate, and poor metabolizers. The maximum dose for poor metabolizers was defined as the dose that would not exceed pimozide concentrations following 10 mg daily in extensive metabolizers and intermediate metabolizers (the current maximum dose in an unselected population). Dose-ranging analyses revealed that 4 mg daily in CYP2D6 poor metabolizers was the maximum dose that would not result in plasma concentrations in excess of those observed in extensive metabolizer and intermediate metabolizer patients receiving 10 mg daily. CYP2D6 genotyping is now consequently recommended in the pimozide product label before exceeding 4 mg of pimozide daily in adults or 0.05 mg/kg/d in children. Previously, dose adjustment was recommended every 3 days to achieve the desired clinical response for all patients. The label was modified to subsequently reflect that pimozide doses should not be increased earlier than 14 days in patients who are known CYP2D6 poor metabolizers. Given the risk of increased pimozide concentrations and longer time to steady state in CYP2D6 poor metabolizers, the FDA has revised the pimozide label to provide clinicians with clearer dosing, titration, and genotype testing recommendations. The new information is intended to enhance therapeutic individualization of pimozide in pediatric and adult patients.

Atrial Selective Effects of Intravenously Administered Vernakalant in Conscious Beagle Dogs

Vernakalant is a relatively atrial-selective antiarrhythmic drug approved for the conversion of recent onset atrial fibrillation in Europe and is under regulatory review in the United States. In this study, we examined the effects of intravenously administered vernakalant (5, 10, and 20 mg/kg) on blood pressure, heart rate, and the electrocardiogram in conscious male beagle dogs and compared them with those of orally administered dl-sotalol (32 mg/kg). Vernakalant had no consistent dose-dependent effects on the heart rate or mean arterial pressure. Although vernakalant inhibits I(Kr), it tended to decrease the QTc interval but only at the top dose and later time points. The most striking effect of vernakalant on the electrocardiogram was a dose-dependent and selective slowing of atrial conduction (P-wave duration), with no effect on ventricular conduction (QRS duration). In contrast, treatment with dl-sotalol resulted in a marked and statistically significant prolongation of PR and QTc intervals with no effect on QRS or P-wave duration, consistent with its known class II and III antiarrhythmic actions. These results provide further evidence that vernakalant is unlikely to alter ventricular refractoriness or conduction at plasma concentrations in excess of those necessary for conversion of atrial fibrillation to sinus rhythm in patients.

Pharmacokinetics of levodopa

This paper reviews the clinically relevant determinants of levodopa peripheral pharmacokinetics and main observed changes in the levodopa concentration-effect relationship with Parkinson's disease (PD) progression. Available clinically practical strategies to optimise levodopa pharmacokinetics and pharmacodynamics are briefly discussed. Levodopa shows particular pharmacokinetics including an extensive presystemic metabolism, overcome by the combined use of extracerebral inhibitors of the enzyme L: -amino acid decarboxylase and rapid absorption in the proximal small bowel by a saturable facilitated transport system shared with other large neutral amino acids. Drug transport from plasma to the brain is mediated by the same carriers operating in the intestinal mucosa. The main strategies to assure reproducibility of both intestinal absorption and delivery to the brain, and the clinical effect include standardization of levodopa dosing with respect to meal times and a controlled dietary protein intake. Levodopa plasma half-life is very short, resulting in marked plasma drug concentration fluctuations which are matched, as the disease progresses, to swings in the therapeutic response ("wearing-off" phenomena). "Wearing-off" phenomena can also be associated, at the more advanced disease stages, with a "negative", both parkinsonism-exacerbating and dyskinetic effect of levodopa at low, subtherapeutic plasma concentrations. Dyskinesias may also be related to high-levodopa, excessive plasma concentrations. Recognition of the different levodopa toxic response patterns can be difficult on a clinical basis alone and simultaneous monitoring of the levodopa concentration-effect relationship may prove useful to disclose the underlying mechanism and in planning the correct management. Clinically practical strategies to optimise levodopa pharmacokinetics, and possibly its therapeutic response, include liquid drug solutions, controlled release formulations and the use of inhibitors of levodopa metabolism. Unfortunately, these attempts have proved so far only partly successful, due to the complex alterations in cerebral levodopa kinetics which accompany the progressive degeneration of the nigrostriatal dopaminergic system in PD patients.

Toxidermie folliculaire à la ciclosporine

Cyclosporine is one of the immunosuppressant agents most widely used in the prevention and treatment of organ transplant rejection and also in autoimmune diseases. Many cutaneous side effects have been described with oral cyclosporine, mainly in transplant recipients, for example, hypertrichosis, gingival hyperplasia and viral skin infections. Here, we report an unusual follicular eruption induced by this drug. A 22-year-old man presenting cystic fibrosis received a double-lung graft in January 2005. Six weeks later, he developed a subacute eruption of follicular papules, not highly pruritic, located mainly on the trunk, the extensor surfaces of the limbs and the face. Diagnosis of cyclosporine-induced follicular eruption was adopted on the basis of the histological and microbiological findings. Complete regression was obtained after switching to tacrolimus. Three similar cases were previously reported characterized by typical follicular changes different from those observed in hypertrichosis or pilar keratosis. This rare cutaneous side effect may be explained by the direct action of cyclosporine on the pilosebaceous unit: this drug is known to extend the anagen phase of the follicular cycle and to induce toxic follicular dystrophy at higher tissue concentrations. This particular toxicity is usually seen after many months of treatment. In our patient, the time to onset was shorter, probably due to occasionally excessive plasma concentrations.

Tipranavir: a protease inhibitor from a new class with distinct antiviral activity.

Tipranavir (TPV) is the first of a new class of non-peptidic protease inhibitors (NPPIs). It is a sulphonamide-containing dihydropyrone, which is highly selective for the HIV protease enzyme and demonstrates potent in vitro activity against wild-type HIV-1 and HIV-2. The IC90 for TPV was 0.1 microM against clinical HIV isolates. Since CYP3A is the major cytochrome P450 isoform for the phase I metabolism of TPV, its exposure is markedly enhanced in the presence of ritonavir (RTV). In one clinical study, using the new self emulsifying drug delivery system (SEDDS) formulation of TPV, plasma concentrations in excess of 20 microM were maintained for 12 hours, allowing for twice-a-day dosing following administration of TPV 300 mg/RTV(r) 200 mg twice a day. The 20 microM target represents 10-fold the IC90 for multiple protease inhibitor (PI)-resistant strains. Both in vitro data and pharmacokinetic results indicate that TPV will be active in vivo against PI-resistant viruses, when given twice a day in combination with low dose RTV. Of 105 HIV viral isolates taken from patients who had been heavily pretreated with PI-based regimens: 90% were fully susceptible to TPV; 8% exhibited intermediate resistance; and 2% were more than 10-fold resistant. In patients who had failed at least two PI-based regimens, only 12.2% of the HIV isolates exhibited four to 10-fold reduced susceptibility to TPV after one year of treatment with a regimen containing the NPPI (Study BI1182.2). A reduction of approximately 1.5 log10 copies/mL in the plasma viral load (pVL) was observed in treatment-naive patients after 15 days of monotherapy with TPV (300 or 1200 mg twice a day) co-administered with RTV (200 mg twice a day) (TPV/r) in a dose-ranging study (Study BI1182.3). The safety and efficacy of TPV (500 or 1250 mg) plus ritonavir (100 mg twice a day) plus two new nucleoside reverse transcriptase inhibitors (NRTIs) was studied in patients failing their first PI-containing regimen (Study BI1182.4). Similar decreases in pVLs (1.44-1.79 log10 copies/mL) were observed after 16 weeks of treatment with either dose of TPV/r. Two doses of TPV/r plus efavirenz (EFV) and a new NRTI have been studied in non-nucleoside reverse transcriptase inhibitor (NNRTI)-naive patients who had failed two or more PI-containing regimens (BI1182.2). Between 50% and 78.9% of patients maintained a pVL < 50 copies/mL for 48 weeks. Clinical studies have shown that TPV/r-associated adverse events are generally gastrointestinal-associated, transient and mild. A phase II study will define the optimal dose of TPV/r for highly treatment-experienced patients. The safety and efficacy of this dose of TPV/r will be evaluated in two phase III studies that will enroll more than 1300 patients worldwide. Tipranavir's robust activity against PI-resistant strains results from its molecular flexibility, which allows it to fit into the active pocket of the protease enzyme in viruses that have become resistant to other PIs.

Flavopiridol, the first cyclin-dependent kinase inhibitor to enter the clinic: current status

This review focuses on the clinical development of the prototype broad spectrum inhibitor of cyclin-dependent kinases (CDKs), flavopiridol, now undergoing Phase II single-agent trials and Phase I combination trials (with paclitaxel and cisplatin). Preclinically, flavopiridol is a potent inhibitor of CDKs 1, 2 and 4 in cell-free assays (IC50 in the region of 100 nM) and tumour cell growth in vitro (typical IC50 in the region of 100 nM). The drug showed in vivo antitumour activity (using iv., ip. or oral dosing) against a variety of human tumour xenografts, especially when administered on a regular daily, rather than weekly, schedule and most notably against prostate carcinoma, head and neck cancer, non-Hodgkin’s lymphoma and leukaemia. The major toxicities observed in rodents were on the bone marrow and gastrointestinal tract. Pharmacokinetics were linear with dose and with a bi-exponential decline both in rodents and man. Oral bioavailability in rodents is in the region of 20%. Glucuronidation appears to be the major route of metabolism. Single-agent clinical trials have mainly used a 72 h continuous infusion schedule. Dose-limiting toxicities were diarrhoea and hypotension. Plasma concentrations in excess of those required for in vitro enzyme or cell growth inhibition are achievable. While there has been some evidence of single-agent antitumour activity (partial responses in a patient with renal cancer and another with gastric cancer), ongoing combination studies, especially with paclitaxel, where preclinical synergistic antitumour effects are observed, are promising. Doubt as to whether CDKs are the sole target responsible for the drug’s antitumour effects have been raised by preclinical observations of apoptosis of non-cycling cells, effects on endothelial cells and non-CDK proteins, such as aldehyde dehydrogenase and glycogen phosphorylase, potent effects on PTEFb and transcription and its ability to directly interact with DNA.

No acute antimigraine efficacy of CP-122,288, a highly potent inhibitor of neurogenic inflammation: Results of two randomized, double-blind, placebo-controlled clinical trials

CP-122,288 is a highly potent inhibitor of neurogenic plasma extravasation in animal models at doses without vasoconstrictor effect. We evaluated the acute antimigraine efficacy of intravenous and oral CP-122,288 in two double-blind studies. In a crossover design, patients randomly received 31.25 microg of CP-122,288 intravenously, placebo, or both. In the oral study, patients received placebo or one of four doses of CP-122,288 between 3.125 and 312.5 microg, using a novel "up and down" design for randomization. Both studies were stopped prematurely when target efficacy could not be achieved. Responder rates were 29% for CP-122,288 versus 30% for placebo (difference, -1%; 95% CI, -24-22%; intravenous study) and an overall rate of 25% for CP-122,288 versus 0% for placebo (difference, 25%; 95% CI; 10-40%; oral study). CP-122,288 was not clinically effective at doses and plasma concentrations in excess of those required to inhibit neurogenic plasma extravasation in animals. Neurogenic plasma extravasation is unlikely to play a crucial role in the pathophysiology of migraine headache.

No acute antimigraine efficacy of CP‐122,288, a highly potent inhibitor of neurogenic inflammation: Results of two randomized, double‐blind, placebo‐controlled clinical trials

CP-122,288 is a highly potent inhibitor of neurogenic plasma extravasation in animal models at doses without vasoconstrictor effect. We evaluated the acute antimigraine efficacy of intravenous and oral CP-122,288 in two double-blind studies. In a crossover design, patients randomly received 31.25 μg of CP-122,288 intravenously, placebo, or both. In the oral study, patients received placebo or one of four doses of CP-122,288 between 3.125 and 312.5 μg, using a novel “up and down” design for randomization. Both studies were stopped prematurely when target efficacy could not be achieved. Responder rates were 29% for CP-122,288 versus 30% for placebo (difference, −1%; 95% CI, −24–22%; intravenous study) and an overall rate of 25% for CP-122,288 versus 0% for placebo (difference, 25%; 95% CI; 10–40%; oral study). CP-122,288 was not clinically effective at doses and plasma concentrations in excess of those required to inhibit neurogenic plasma extravasation in animals. Neurogenic plasma extravasation is unlikely to play a crucial role in the pathophysiology of migraine headache. Ann Neurol 2000;47:238–241

NPS 1506, a novel NMDA receptor antagonist and neuroprotectant. Review of preclinical and clinical studies.

NPS 1506 is a moderate affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. NPS 1506 is neuroprotective in rodent models of ischemic stroke, hemorrhagic stroke, and head trauma, with a 2-hr window of opportunity. Neuroprotectant doses of NPS 1506 ranged from approximately 0.1-1.0 mg/kg, with peak plasma concentrations ranging from 8-80 ng/mL. Even at doses producing behavioral toxicity, NPS 1506 did not elicit MK-801-like behaviors, did not generalize to phencyclidine (PCP), and did not elicit neuronal vacuolization. In a Phase I study, intravenous (i.v.) doses of NPS 1506 from 5-100 mg were well tolerated and provided plasma concentrations in excess of those required for neuroprotection in rodents. Adverse events at the 100-mg dose included mild dizziness and lightheadedness, and mild to moderate ataxia. Neither PCP-like psychotomimetic effects nor cardiovascular effects were noted. The long plasma half-life of NPS 1506 (approximately 60 hr) suggests that a single i.v. dose will provide prolonged neuroprotection in humans.

The Role of Metabolites of Antidepressants in the Treatment of Depression.

Recognition of the role of active metabolites in mediating therapeutic and/or adverse effects of many antidepressants is an important part of understanding the mechanisms of action of these medications. While virtually all antidepressants except lithium undergo extensive hepatic metabolism, the profile of activity of the resulting breakdown products varies considerably.The metabolites of some antidepressants share the primary biochemical actions of their parent compounds and appear to contribute to the therapeutic efficacy of those medications. Examples of this are the tricyclic antidepressant (TCA) nor-triptyline, the selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) fluoxetine and the serotonin-noradrenaline (norepinephrine) reuptake inhibitor venlafaxine. Less commonly, the activity of the primary metabolite may differ from that of the parent drug. An example is clomipramine. This drug is a potent serotonin reuptake blocking TCA, but its demethyl-metabolites are noradrenaline reuptake inhibitors. On the other hand, a number of effective anti-depressants, including most of the SSRIs other than fluoxetine, lack active metabolites.On the negative side, antidepressant metabolites may add to the adverse effect burden presented by their drugs of origin. At sufficiently high doses, the amphetamines resulting from the metabolism of some monoamine oxidase inhibitors, e.g. selegiline (deprenyl), may directly produce toxicity from the pharmacodynamic interaction with the parent antidepressant. While hydroxy-nortriptyline produces lesser anticholinergic effects than its parent compound, this metabolite may block the therapeutic action of nortriptyline when present in high concentrations. Excessive plasma concentrations of the major metabolite of amfebutamone (bupropion) have been associated with nonresponse and clinical worsening in some patients.Amfebutamone also illustrates the importance of pharmacokinetic factors in determining the magnitude of the influence of metabolites on antidepressant action. Active metabolites that have long elimination half-lives may predominate over the parent compound in plasma and CSF, exerting considerable clinical impact. With several of the newer drugs, notably amfebutamone, venlafaxine and nefazodone, the presence of active metabolites with half-lives approaching 1 day suggests that once-daily administration may be sufficient.The formation of most antidepressant metabolites is under strong genetic control and the metabolites themselves often exert effects on hepatic enzyme systems. This can lead to the possibility of drug-drug interactions. A key example is norfluoxetine, which is associated with potent inhibition of the cytochrome P450 isozyme 2D6 (and, consequently, reduced metabolism of drugs such as TCAs). This effect lasts for weeks even after fluoxetine discontinuation, due to the fact that norfluoxetine has a half-life of up to 2 weeks.The clearance of most antidepressant metabolites is ultimately dependent on elimination by the kidneys. Therefore, these substances tend to accumulate in states of reduced renal function, including normal aging. The relative increase in TCA hydroxy-metabolite concentrations in the elderly may contribute to the cardiovascular and other toxicities of these antidepressants in this vulnerable patient population.Attention to the existence and implications of active metabolites from the earliest stages of antidepressant drug development may help optimise the benefit: risk ratio of this valuable class of psychotropic medications.

Pharmacokinetic optimisation in the treatment of Parkinson's disease.

The current symptomatic treatment of Parkinson's disease mainly relies on agents which are able to restore dopaminergic transmission in the nigrostriatal pathway, such as the dopamine precursor levodopa or direct agonists of dopamine receptors. Ancillary strategies include the use of anticholinergic and antiglutamatergic agents or inhibitors of cerebral dopamine catabolism, such as monoamine oxidase type B inhibitors. Levodopa is the most widely used and effective drug. Its peculiar pharmacokinetics are characterised by an extensive presystemic metabolism, overcome by the combined use of extracerebral inhibitors of the enzyme aromatic-amino acid decarboxylase and rapid adsorption in the proximal small bowel by a saturable facilitated transport system shared with other large neutral amino acids. Drug transport from plasma to the brain is mediated by the same carriers operating in the intestinal mucosa. The main strategies to assure reproducibility of both drug intestinal absorption and delivery to the brain and clinical effect include standardisation of levodopa administration with respect to meal times and a controlled dietary protein intake. The levodopa plasma half-life is very short, resulting in marked plasma drug concentration fluctuations which are matched, as the disease progresses, with swings in the therapeutic response ('wearing-off' phenomena). 'Wearing-off' phenomena can be also associated, at the more advanced disease stages with a 'negative', both parkinsonism-exacerbating and dyskinetic effect of levodopa at subtherapeutic plasma concentrations. Dyskinesias may be also related to high-levodopa, excessive plasma concentrations. Recognition of the different levodopa toxic response patterns can be difficult on a clinical basis alone, and simultaneous monitoring of levodopa concentration-effect relationships may prove useful to disclose the underlying mechanism and in planning the correct pharmacokinetic management. Controlled-release levodopa formulations have been developed in an attempt to smooth out fluctuations in plasma profiles and matched therapeutic responses. The delayed levodopa absorption and lower plasma concentrations which characterise controlled-release formulations compared with standard forms must be taken into account when prescribing dosage regimens and can be complicating factors in the management of the advanced disease stages. The pharmacokinetic and pharmacodynamic characterisation of the other antiparkinsonian agents is hampered by the lack of sensitive and specific analytical methods to measure their very low plasma drug concentrations and by the difficulty in quantitatively assessing overall moderate drug clinical effects. In clinical practice an optimal dosage schedule is still generally found for each patient on an empirical basis. Future strategies should focus on the search for pharmacological agents with a better kinetic profile, particularly a higher and reproducible bioavailability and a predictable relationship between plasma drug concentration and clinical response. Treatments aimed not only at controlling the symptoms, but also at slowing the neurodegenerative process, are currently under intensive investigation.

Pharmacokinetics of Intravenous Milrinone in Patients Undergoing Cardiac Surgery

Milrinone is a phosphodiesterase inhibitor with positive inotropic and vasodilator effects that are useful in the treatment of ventricular dysfunction after cardiac surgery. However, the pharmacokinetics of the drug have been investigated only in healthy volunteers and in patients with chronic congestive heart failure. This study investigates the pharmacokinetics of milrinone in adult cardiac surgical patients after cardiopulmonary bypass. Milrinone was administered to 25 patients just before or immediately after separation from cardiopulmonary bypass. Arterial blood was sampled over the next 16 h and milrinone plasma concentrations were determined by high-performance liquid chromatography. Data were analyzed by extended nonlinear least-squares regression. The relation between milrinone plasma concentration and hemodynamic effect was examined in an additional 11 patients who had cardiac indices less than 2.5 l.min-1.m-2 immediately after separation from cardiopulmonary bypass. Milrinone was administered and plasma concentrations were related to changes in cardiac index during the next 10 min. A milrinone dose of 50 micrograms/kg in conjunction with an infusion of 0.5 micrograms.kg-1.min-1 consistently maintained plasma concentrations in excess of 100 ng/ml. A triexponential equation describing the plasma concentration as a function of time was used to describe the data. Central-compartment volume was 102 ml/kg, volume of distribution was 1,698 ml/kg, and elimination clearance was 1.88 ml.kg-1.min-1. Pharmacokinetic parameters were independent of dose. The relation between plasma concentration and percentage increase in cardiac index could be described by a sigmoidal curve with the plasma concentration associated with a 50% increase in cardiac index equal to 167 ng/ml. A milrinone dose of 50 micrograms/kg with an infusion at 0.5 micrograms.kg-1.min-1 maintains plasma concentrations at or above the threshold of therapeutic effects.

Pharmacokinetics of Aprotinin in Preoperative Cardiac Surgical Patients

Aprotinin, a polypeptide protease inhibitor, reduces bleeding and transfusion requirements during cardiac surgery. To investigate aprotinin's pharmacokinetics, we administered therapeutic doses to patients scheduled to undergo cardiac surgery. Preoperative doses of 500,000 and 1,000,000 kallikrein inhibitor units (KIU) were administered as an infusion over 30 min to 28 patients, and plasma concentrations were measured for 48 h. Aprotinin concentrations were determined using a sandwich-enzyme-linked immunosorbent assay. A three-compartment model was fit to the measured aprotinin concentrations using extended nonlinear least-squares regression. Plasma aprotinin concentrations at the end of the 30-min infusion were 147 +/- 61 KIU/ml for the 1,000,000-KIU dose and 60 +/- 19 KIU/ml for the 500,000-KIU dose. Elimination clearance was 35.5 ml/min, and volume of distribution at steady state was 26.5 l. A 2,000,000-KIU dose is needed to produce the plasma concentration of 200 KIU/ml associated with kallikrein inhibition. Plasma concentrations in excess of 50 KIU/ml, the concentration required to inhibit plasmin, can be achieved by a significantly smaller dose. Based on the derived pharmacokinetic parameters, an infusion model was developed to appropriately administer and maintain effective plasma concentrations of aprotinin, depending on the plasma concentration desired and the target proteases to be inhibited.

Evaluation of sufentanil anesthesia obtained by a computer-controlled infusion for cardiac surgery.

To determine the "therapeutic window" for sufentanil in cardiac surgery, nine hemodynamically stable, lightly premedicated patients were anesthetized with sufentanil to undergo coronary artery bypass grafting. Sufentanil was administered by a computer-controlled infusion pump programmed with an infusion scheme based on average sufentanil pharmacokinetics. The presence of somatic or hemodynamic responses to tracheal intubation, skin incision, sternotomy, and during electrocauterization, to isolate the internal mammary artery were correlated with plasma concentrations of sufentanil. The concentration versus response/no response relationships were analyzed by application of the logistic (Hill) equation. The CP50 of sufentanil to suppress responses to intubation, incision, and sternotomy was 7.1 ng/mL and for mediastinal dissection it was 12.7 ng/mL. Interpatient variability was as much as that reported previously for other opioids in other types of surgical patients. The computer-controlled infusion scheme consistently produced sufentanil plasma concentrations in excess of the target concentrations, but there was a strong correlation between target and actual sufentanil concentrations (r = 0.88, P < 0.05).

Rapid Removal of Heparin from Plasma by Affinity Filtration

Coagulation test abnormalities caused by the presence of heparin are not uncommon and frequently result in additional laboratory investigation and unwarranted therapy. Methods to neutralize the effect of heparin include the addition of polyanions, enzymes, or resins to the clinical samples. These techniques are time consuming and cumbersome and produce inconsistent results. As an alternative, use of a positively charged porous filtration medium to remove heparin from the sample is described. The filtration procedure requires less than one minute and provides a consistent and total removal of heparin at plasma concentrations in excess of therapeutic levels. Chromogenic substrate analysis for residual heparin activity confirms total removal of the drug at levels greater than one unit/mL of plasma. In clinical trials, filtration normalized the activated partial thromboplastin time (APTT) of all patient specimens containing heparin (n = 41). Filtration did not shorten the coagulation assay times of patients receiving warfarin (n = 36) or of those with a variety of acquired coagulopathies (n = 14). The level of coagulation factors adsorbed to the filter medium compares favorably to that adsorbed by other commercially available heparin-binding resins. Routine use of the filter in the clinical laboratory allows for the rapid and definitive identification of heparin in specimens with prolonged coagulation times and provides clinically meaningful data.

TEMAZEPAM ABSORPTION IN PATIENTS BEFORE SURGERY

We have compared the rates of absorption and efficacies of temazepam 30 mg in elixir and capsule formulations in 100 patients before surgery. Both formulations provided anxiolysis and sedation, but there was wide variation in plasma concentrations of temazepam between individuals and between formulations. The presence or absence of anxiety did not influence the absorption of the preparations. It is suggested that plasma concentrations in excess of 200 ng ml-1 are required for sedation and anxiolysis, and that this may be achieved more reliably using the elixir formulation.

Potential Use of Long-Acting Injectable Oxytetracycline for Treatment of Chlamydiosis in Goffin's Cockatoos

To determine the potential use of parenteral therapy in the treatment of chlamydiosis in psittacine birds, the disposition and toxicity of a long-acting oxytetracycline (OTC) was evaluated in Goffin's cockatoos. Following intramuscular and subcutaneous administration of 50 to 100 mg/kg body weight, plasma OTC concentrations of 7 to 15 micrograms/ml were obtained 3 hr following injection and declined with a terminal half-life between 8.9 to 14.7 hr. Plasma concentrations in excess of 1.0 microgram/ml were maintained for 48 to 68 hr. Multiple-dose treatment of 100 mg/kg subcutaneously every 3 days for 30 days caused focal necrosis and scabs at the injection site but no other clinical or serological evidence of adverse effects. Long-term treatment did not result in accumulation or alteration in the disposition of OTC. Based on this study, a dosage regimen of 50 to 100 mg/kg of OTC subcutaneously every 2-3 days would safely maintain plasma concentrations in excess of 1.0 microgram/ml and could potentially be used as an alternative to medicated feeds or daily oral dosing regimens for the treatment of chlamydiosis in psittacine birds.