Management of erythropoietic protoporphyria with cholestatic liver disease: A case report

Erythropoietic protoporphyria (EPP) is a rare metabolic disease of the heme biosynthetic pathway where an enzymatic dysfunction results in protoporphyrin IX (PPIX) accumulation in erythroid cells. The porphyrins are photo-reactive and are responsible for severe photosensitivity in patients, thus drastically decreasing their quality of life. The liver eliminates PPIX and as such, the main and rare complication of EPP is progressive cholestatic liver disease, which can lead to liver failure. The management of this complication is challenging, as it often requires a combination of approaches to promote PPIX elimination and suppress the patient's erythropoiesis. Here we described a 3-year follow-up of an EPP patient, with three episodes of liver involvement, aggravated by the coexistence of a factor VII deficiency. It covers all the different types of intervention available for the management of liver disease, right through to successful allogeneic hematopoietic stem cell transplantation.

Pregnancy outcomes in women with primary Sjögren's syndrome: an analysis of data from the multicentre, prospective, GR2 study

BACKGROUND

Adverse pregnancy outcomes in women with primary Sjögren's syndrome have only been evaluated retrospectively using heterogeneous methods and with contradictory results. We aimed to describe adverse pregnancy, delivery, and birth outcome risks in pregnant women with primary Sjögren's syndrome compared with those of a matched general population in France, and to identify factors predictive of disease flares or adverse pregnancy outcomes.

METHODS

We conducted a multicentre, prospective, cohort study in France using the GR2 (Groupe de Recherche sur la Grossesse et les Maladies Rares) registry. Women from the GR2 study were eligible if they had conceived before March, 2021, had primary Sjögren's syndrome according to the American College of Rheumatology and European Alliance of Associations for Rheumatology (EULAR) 2016 classification criteria, and had an ongoing pregnancy at 12 weeks of gestation. In women who entered in the registry with pregnancies before 18 weeks of gestation, we sought to identify factors associated with primary Sjögren's syndrome flare (≥3-point increase in EULAR Sjögren's Syndrome Disease Activity Index [ESSDAI] score) or adverse pregnancy outcomes (fetal or neonatal death, placental insufficiency leading to a preterm delivery [<37 weeks of gestation], or small-for-gestational-age birthweight). A matched controlled study compared adverse pregnancy, delivery, and birth outcome rates between pregnant women with primary Sjögren's syndrome from the GR2 registry and matched controls from the general population included in the last French perinatal survey (Enquête Nationale Périnatale 2016).

FINDINGS

1944 pregnancies were identified in the GR2 cohort, of which 106 pregnancies in 96 women with primary Sjögren's syndrome were included in this analysis. The median age at pregnancy onset was 33 years (IQR 31-36). 87 (83%) of 105 pregnancies (with ethnicity data) were in White women, 18 (17%) were in Black women; 92 (90%) of 102 had previous systemic activity (ESSDAI score of ≥1; data missing in four pregnancies), and 48 (45%) of 106 had systemic activity at inclusion. Of 93 pregnancies included at week 18 of gestation or earlier, primary Sjögren's syndrome flares occurred in 12 (13%). No baseline parameters were associated with primary Sjögren's syndrome flare. Four twin pregnancies and one medical termination were excluded from the adverse pregnancy outcome analysis; of the remaining 88, adverse pregnancy outcomes occurred in six (7%). Among pregnancies in women with data for antiphospholipid antibodies (n=55), antiphospholipid antibody positivity was more frequent among pregnancies with adverse outcomes (two [50%] of four pregnancies) compared with those without adverse outcomes (two [4%] of 51 pregnancies; p=0·023). Anti-RNP antibody positivity was also more frequent among pregnancies with adverse outcomes than those without, although this was not statistically significant. In the matched controlled study, adverse pregnancy outcomes occurred in nine (9%) of 105 pregnancies in women with primary Sjögren's syndrome and 28 (7%) of the 420 matched control pregnancies; adverse pregnancy outcomes were not significantly associated with primary Sjögren's syndrome (odds ratio 1·31, 95% CI 0·53-2·98; p=0·52).

INTERPRETATION

Pregnancies in women with primary Sjögren's syndrome had very good prognoses for mothers and fetuses, with no overall increase in adverse pregnancy outcome risk compared with the general population. Women with antiphospholipid antibodies or anti-RNP antibodies require close monitoring, because these factors might be associated with a higher risk of adverse pregnancy outcomes.

FUNDING

Lupus France, Association des Sclérodermiques de France, Association Gougerot Sjögren, Association Francophone Contre la Polychondrite Chronique Atrophiante, AFM-Telethon, Société Nationale Française de Médecine Interne, Société Française de Rhumatologie, Cochin Hospital, French Health Ministry, Fondation for Research in Rheumatology, Association Prix Véronique Roualet, Union Chimique Belge.

[Characteristics of patients admitted from emergency units in 18 internal medicine departments and organisation of these departments: A cross sectional study from SNFMI (SiFMI study group) in 2015]

INTRODUCTION

Patients admitted from emergency units represent a large portion of the population in internal medicine departments. The aim of this study is to identify characteristics of patients and organization of these departments.

METHODS

Between June 29th and July 26th 2015, voluntary internal medicine departments from the SiFMI group prospectively filled anonymized internet forms to collect data of each patients admitted in their ward from emergency units, during seven consecutive days.

RESULTS

Three hundred and sixty-five patients from emergency departments were admitted in 18 internal medicine inpatients departments, totalling 1100 beds and 33,530 annual stays, 56% of them for emergency units inpatients. Mean age was 68 years, 54% were women, mean Charlson score was 2.6 and 44% of the patients took at least three drugs. Main causes of hospitalization were infectious (29%) and neurological (17%) diseases. Mean length of stay was 9.2 days. The medical team was composed by a median value of 4,5 [2,75-6,25] senior full-time equivalents, 86% were internists. Each department except one received residents, two third of them were from general medicine.

CONCLUSION

This study highlights a high organizational variability among internal medicine departments and patients, and sets internal medicine as a specialty with a great capacity to achieve an integrative/comprehensive management of patients and to offer a comprehensive basis for physicians in training.

Tocilizumab in treatment-naïve patients with Takayasu arteritis: TOCITAKA French prospective multicenter open-labeled trial

Objectives

To assess long-term efficacy of tocilizumab in treatment-naive patients with Takayasu arteritis (TAK).

Methods

Prospective open-labeled trial in naïve patients with TAK who received steroids at the dose of 0.7 mg/kg/day and 7 infusions of 8 mg/kg/month of tocilizumab. The primary endpoint was the number of patients who discontinued steroids after 7 infusions of tocilizumab. Secondary endpoints included disease activity and the number of relapses during 18-month follow-up.

Results

Thirteen patients with TAK were included, with a median age of 32 years [19–45] and 12 (92%) females. Six (54%) patients met the primary end-point. A significant decrease of disease activity was observed after 6 months of tocilizumab therapy: decrease of median NIH scale (3 [3, 4] at baseline, versus 1 [0–2] after 6 months; p < 0.001), ITAS-2010 score (5 [2–7] versus 3 [0–8]; p = 0.002), and ITAS-A score (7 [4–10] versus 4 [1–15]; p = 0.0001)]. During the 12-month follow-up after tocilizumab discontinuation, a relapse occurred among 5 patients (45%) out of 11 in which achieved remission after 6 months of tocilizumab.

Conclusion

Tocilizumab seems an effective steroid sparing therapy in TAK, but maintenance therapy is necessary.

Trial registration

ClinicalTrials.gov NCT02101333. Registered on 02 April 2014.

Comparative tissue uptake and cellular deposition of three different plutonium chemical forms in rats

PURPOSE

To evaluate the influence of the chemical form of plutonium (Pu) on its distribution in tissues and within liver cells populations.

MATERIALS AND METHODS

Groups of male Sprague-Dawley rats were contaminated by intravenous injection of either Pu citrate, Pu nitrate or Pu phytate. Pu content was determined in various tissues at different times after injection. Pu liver distribution was analysed by autoradiography and after cellular separation.

RESULTS

Biokinetic studies indicate that Pu citrate and Pu nitrate predominantly retained in the skeleton within the first hours after injection, whereas most of the Pu was in the liver after injection of Pu phytate. Autoradiographs showed that Pu citrate was homogeneously distributed in the liver, while Pu nitrate accumulated into 'hot points'. Pu phytate showed an intermediate distribution pattern. Hepatic cell separation revealed a difference of uptake between the two cell types depending on the chemical form of injected Pu, and on the time after contamination.

CONCLUSIONS

Distinct Pu behaviour was observed for biokinetics, retention and liver distribution. The large differences noted between citrate, nitrate and phytate might be explained by differences in systemic and hepatic transport.

In vivo measurement of Pu dissolution parameters of MOX aerosols and related uncertainties in the values of the dose per unit intake

The aim of this study was to compare dissolution parameter values for Pu from industrial MOX with different Pu contents. For this purpose, preliminary results obtained after inhalation exposure of rats to MOX containing 2.5% Pu are reported and compared to those obtained previously with MOX containing 5% Pu. Dissolution parameter values appear to increase when the amount of Pu decreases. Rapid fractions, f(r), of 4 x 10(-3) (s.d. = 2 x 10(-3)) and 1 x 10(-3) (s.d. = 6 x 10(-4)) and slow dissolution rates, s(s) of 2 x 10(-4) d(-1) (standard deviation, sigma = 5 x 10(-5)) and 5 x 10(-5) d(-1) (sigma = 1 x 10(-5)) were derived for MOX containing 2.5 and 5% of Pu, respectively. Simulations were performed to assess uncertainties on dose due to experimental errors. The relative standard deviations of the dose per unit intake (DPUI) due to f(r) (4-8%), are far less than those due to s(s) (about 20%), which is the main parameter altering the dose. Although quite different dissolution parameter values were derived, similar DPUIs were obtained for MOX aerosols containing 2.5 and 5% Pu which appear close to that for default Type S values.

Uncertainties on the committed equivalent dose to the thyroid as a function of age for different iodine isotopes

This study compares uncertainties of equivalent doses after internal contamination by 125I, 129I or 131I. Uncertainties were calculated using reported distributions of physiological parameters and Monte Carlo simulation. In adults, uncertainties increase from 131I to 125I and 129I with 1% of the population receiving 3.9, 4.0 and 7.2 times the median dose for the respective isotopes. In newborns, these values were 7.5, 12.3 and 19.0 for 131I, 125I and 129I respectively. The ratio of the beta dose delivered to the epithelium versus a homogeneous distributed dose was estimated for different iodine concentrations in colloid, epithelium and interstitium. In adults, for 131I, about 40% of the beta dose was delivered to the epithelial cells, whereas this fraction varied depending on the concentration for 125I and 129I, i.e. 20-30% at a relative epithelial concentration of 20% and 7-14% at a concentration of 3%. Small variations were observed depending on age.

In vivo detection of striatal dopamine release during reward: a PET study with [(11)C]raclopride and a single dynamic scan approach

A new simple method is proposed to detect, using PET and [(11)C]raclopride, changes in striatal extracellular dopamine concentration during a rewarded effortful task. This approach aimed to increase the sensitivity in detection of these effects. It requires a single-dynamic PET study and combines the classic kinetic compartmental model with the general linear model of SPM to provide statistical inference on changes in [(11)C]raclopride time-activity curve due to endogenous dopamine release during two short periods of activation. Kinetic simulations predicted that 100% dopamine increase during two 5-min periods starting at 30 and 60 min after the injection can be detected. Moreover the effects of dopamine release on the [(11)C]raclopride time-activity-curve are different from those induced by CBF increase. These simulated curves were used to construct the statistical linear model and to test voxel-by-voxel in healthy subjects the hypothesis that dopamine is released in the ventral striatum during periods of unexpected monetary gains, but not during periods of unexpected monetary loss. The experimental results are in line with the expected results although the amplitude of the effects due to dopamine release is moderate. The advantages and the limits of this method as well as the relevance of the results for dopamine involvement in reward processing are discussed.

[Specific parameters for the calculation of dose after aerosol inhalation of transuranium elements]

A review on specific parameter measurements to calculate doses per unit of incorporation according to recommendations of the International Commission of Radiological Protection has been performed for inhaled actinide oxides. Alpha activity distribution of the particles can be obtained by autoradiography analysis using aerosol sampling filters at the work places. This allows us to characterize granulometric parameters of "pure" actinide oxides, but complementary analysis by scanning electron microscopy is needed for complex aerosols. Dissolution parameters with their standard deviation are obtained after rat inhalation exposure, taking into account both mechanical lung clearance and actinide transfer to the blood estimated from bone retention. In vitro experiments suggest that the slow dissolution rate might decrease as a function of time following exposure. Dose calculation software packages have been developed to take into account granulometry and dissolution parameters as well as specific physiological parameters of exposed individuals. In the case of poorly soluble actinide oxides, granulometry and physiology appear as the main parameters controlling dose value, whereas dissolution only alters dose distribution. Validation of these software packages are in progress.

[The effect of dose distribution on risk of lung cancer after inhalation of actinide oxides]

The aim of this work was to estimate risk of lung tumour occurrence after inhalation of actinide oxides from published studies and rat studies in progress. For the same delivered dose, the risk increases when homogeneity of irradiation increases, i.e., the number of particles deposited after inhalation increases (small particles and (or) low specific alpha activity). The dose-effect relationships reported appear linear up to a few gray, depending on the aerosol considered, and then the slope decreases. This slope, which corresponds with the risk, can vary over one order of magnitude depending on the aerosol used. An effective threshold at about 1 Gy was not observed for the most homogeneous dose distributions. A dosimetric and biological approach is proposed to provide a more realistic risk estimate.

Myocardial muscarinic receptor upregulation and normal response to isoproterenol in denervated hearts by familial amyloid polyneuropathy

BACKGROUND

Patients with familial amyloid polyneuropathy, a rare hereditary form of amyloidosis, have progressive autonomic neuropathy. The disease usually does not induce heart failure but is associated with sudden death, conduction disturbances, and an increased risk of complications during anesthesia. Although cardiac sympathetic denervation has been clearly demonstrated, the postsynaptic status of the cardiac autonomic nervous system remains unelucidated.

METHODS AND RESULTS

Twenty-one patients were studied (age, 39+/-11 years; normal coronary arteries; left ventricular ejection fraction 68+/-9%). To evaluate the density and affinity constants of myocardial muscarinic receptors, PET with (11)C-MQNB (methylquinuclidinyl benzilate), a specific hydrophilic antagonist, was used. Cardiac beta-receptor functional efficiency was studied by the heart rate (HR) response to intravenous infusion of isoproterenol (5 minutes after 2 mg of atropine, 5, 10, and 15 ng/kg per minute during 5 minutes per step). The mean muscarinic receptor density was higher in patients than in control subjects (B'(max), 35.5+/-8.9 versus 26.1+/-6.7 pmol/mL, P=0.003), without change in receptor affinity. The increase in HR after injection of atropine as well as of MQNB was lower in patients compared with control subjects despite a similar basal HR (DeltaHR after atropine, 11+/-21% versus 62+/-17%; P<0.001), consistent with parasympathetic denervation. Incremental infusion of isoproterenol induced a similar increase in HR in patients and control subjects.

CONCLUSIONS

Cardiac autonomic denervation in familial amyloid polyneuropathy results in an upregulation of myocardial muscarinic receptors but without change in cardiac beta-receptor responsiveness to catecholamines.

Parametric images of the extrastriatal D2 receptor density obtained using a high-affinity ligand (FLB 457) and a double-saturation method

The potential of positron emission tomography for the quantitative estimation of receptor concentration in extrastriatal regions has been limited in the past because of the low density of the D2 receptor sites in these regions and the insufficient affinity of the most widely used radioligands for dopamine receptors. The new method described in this paper permits the estimate of the D2 receptor concentration in the extrastriatal regions using a two-injection protocol and FLB 457, a ligand with a high affinity (20 pmol/L in vitro ) with D2 dopamine receptors. This approach is not valid for the striatal regions because some hypotheses cannot be verified (because of the high receptor concentration in these regions). The experimental protocol includes two injections with ligand doses designed to significantly occupy the extrastriatal receptor sites (approximately 90%), while leaving less than 60% of the receptor sites occupied by the ligand in the striatal regions. The results obtained using this double-saturation method are in line with the concentration estimates previously obtained using the multiinjection approach. The receptor concentration is 2.9 +/- 0.5 pmol/mL in the thalamus, 1.0 +/- 0.2 pmol/mL in the temporal cortex, and 0.35 +/- 0.13 pmol/mL in the occipital cortex. This study provides new arguments supporting the presence of a small receptor-site concentration in the cerebellum, estimated at 0.35 +/- 0.16 pmol/mL The simplicity of the calculation used to estimate the receptor concentration lends itself easily to parametric imaging. The receptor concentration is estimated pixel by pixel, without filtering. This method permits estimation of the extrastriatal D2 receptor concentration using an experimental protocol that can easily be used in patient studies (i.e., single experiment, no blood sampling, short experiment duration).

Absolute quantification by positron emission tomography of the endogenous ligand

The results of several recent papers have shown a significant influence of the endogenous neurotransmitters on the exogenous ligand kinetics measured by positron emission tomography. For example, several groups found that the percentage of D2 receptor sites occupied by the endogenous dopamine ranged from 25% to 40% at basal level. An obvious consequence of this significant occupancy is that the ligand-receptor model parameters, usually estimated by a model that does not take into account the endogenous ligand (EL) kinetics, can be significantly biased. In the current work, the authors studied the biases obtained by using the multiinjection approach. The results showed that in the classical ligand-receptor model, the receptor concentration is correctly estimated and that only the apparent affinity is biased by not taking the EL into account. At present, all absolute quantifications of the EL have been obtained through pharmacologic manipulation of the endogenous transmitter concentration, which is often too invasive a method to be used in patients. A theoretical reasoning showed that a noninvasive approach is necessarily based on both the apparent affinity measurement and on a multiregion approach. The correlation between the receptor concentration and the apparent affinity, previously observed with some ligands, verifies these two conditions; thus, the authors suggest that this correlation could be the result of the EL effect. To test this assumption experimentally, the effect of reserpine-induced dopamine depletion on the interactions between the D2 receptor sites and the FLB 457 is studied. With untreated baboons, the apparent FLB 457 affinity was smaller in the receptor-rich regions (striatum) than in the receptor-poor regions. This discrepancy disappeared after dopamine depletion, strongly suggesting that this affinity difference was related to the EL effect. Therefore, the purpose of the current study was to test the ability to quantify the EL based on the observed correlation between the receptor concentration and the apparent affinity. This approach offers a method for estimating the percentage of receptor sites occupied by the EL and, if its affinity is known, the free EL concentration. From the data obtained using FLB 457 with baboons, the authors found that approximately 53% of the D2 receptor sites are occupied by dopamine in the striatum and that the free dopamine concentration is approximately 120 nmol/L at basal level. This approach is transferable to patients, because the experimental data are obtained without pharmacologically induced modification of the EL.

Wavelet analysis of dynamic PET data: application to the parametric imaging of benzodiazepine receptor concentration

Receptor density and ligand affinity can be assessed using positron emission tomography (PET). Biological parameters (B(max)('), k(1), k(2), k(on)/V(R), k(off)) are estimated using a compartmental model and a multi-injection protocol. Parametric imaging of the ligand-receptor model has been shown to be of special interest to study certain brain disorders. However, the low signal-to-noise ratio in kinetic curves at the pixel level hampers an adequate estimation of model parameters during the optimization procedure. For this reason, mapping requires a spatial filter, resulting in a loss of resolution. Filtering the kinetic curves in the frequency domain using the Fourier transform is not appropriate, because of difficulties in choosing a correct and efficient cutoff frequency. A wavelet-based filter is more appropriate to such tracer kinetics. The purpose of this study is to build up parametric images at the pixel level while conserving the original spatial resolution, using wavelet-based filtering. Data from [(11)C]flumazenil studies, mapping the benzodiazepine receptor density, were used. An invertible discrete wavelet transform was used to calculate the time-frequency signals of the time-concentration PET curves on a pixel-by-pixel basis. Kinetic curves observed from large regions of interest in high and low receptor-density regions were used to calibrate the threshold of wavelet coefficients. The shrunken wavelet coefficients were then transformed back to the original domain in order to obtain the filtered PET signal. Maps of all binding parameters were obtained at the pixel level with acceptable coefficients of variation of less than 30% for the B(max)(') parameter in most of the gray matter. A strong correlation between model parameter estimates using the usual regions of interest and parametric imaging was observed for all model parameters (r = 0.949 for the parameter B(max)(')). We conclude that wavelet-based filters are useful for building binding parameter maps without loss of the original spatial resolution of the PET scanner. The use of the wavelet-based filtering method can be extended far beyond the multi-injection protocol. It is likely to be also effective for other dynamic PET studies.

Quantitation of extrastriatal D2 receptors using a very high-affinity ligand (FLB 457) and the multi-injection approach

The multi-injection approach has been used to study in baboon the in vivo interactions between the D2 receptor sites and FLB 457, a ligand with a very high affinity for these receptors. The model structure was composed of four compartments (plasma, free ligand, and specifically and unspecifically bound ligands) and seven parameters (including the D2 receptor site density). The arterial plasma concentration, after correction for metabolites, was used as the input function. The experimental protocol, which consisted of three injections of labeled and/or unlabeled ligand, allowed the evaluation of all model parameters from a single positron emission tomography experiment. In particular, the concentration of receptor sites available for binding (B'max) and the apparent in vivo FLB 457 affinity were estimated in seven brain regions, including the cerebellum and several cortex regions, in which these parameters are estimated in vivo for the first time (B'max is estimated to be 4.0+/-1.3 pmol/mL in the thalamus and from 0.32 to 1.90 pmol/mL in the cortex). A low receptor density was found in the cerebellum (B'max = 0.39+/-0.17 pmol/mL), whereas the cerebellum is usually used as a reference region assumed to be devoid of D2 receptor sites. In spite of this very small concentration (1% of the striatal concentration), and because of the high affinity of the ligand, we demonstrated that after a tracer injection, most of the PET-measured radioactivity in the cerebellum results from the labeled ligand bound to receptor sites. The estimation of all the model parameters allowed simulations that led to a precise knowledge of the FLB 457 kinetics in all brain regions and gave the possibility of testing the equilibrium hypotheses and estimating the biases introduced by the usual simplified approaches.

Increased myocardial muscarinic receptor density in idiopathic dilated cardiomyopathy: an in vivo PET study

BACKGROUND

Congestive heart failure is associated with decreased stimulated myocardial adenylate cyclase activity, increased Gi-binding protein, attenuated parasympathetic tone, and increased modulation of beta-adrenergic inotropic left ventricular stimulation by parasympathetic agonists. Despite these abnormalities, changes in the density or affinity of ventricular muscarinic receptors have not been demonstrated in patients.

METHODS AND RESULTS

The density and affinity constants of myocardial muscarinic receptors were evaluated noninvasively by means of positron emission tomography with 11C-MQNB (methylquinuclidinyl benzilate), a specific hydrophilic antagonist, in 20 patients with congestive heart failure due to idiopathic dilated cardiomyopathy (mean left ventricular ejection fraction, 22+/-9%) and compared with values in 12 normal subjects. The mean receptor concentration was significantly higher in patients than in control subjects (B'max, 34.5+/-8.9 versus 25+/-7.7 pmol/mL, P<.005), with no changes in affinity constants. The change in heart rate after injection of 0.6 mg of cold MQNB was lower in patients than in control subjects (34+/-20% versus 55+/-36%, P<.05), and receptor density correlated negatively with maximal heart rate in the patients (r=.45, P<.05).

CONCLUSIONS

Congestive heart failure is associated with an upregulation of myocardial muscarinic receptors. This may be an adaptive mechanism to beta-agonist stimulation and should increase the number of potential targets for pharmacological intervention.

Pharmacokinetic considerations in the PET and SPECT evaluation of CNS receptors

Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are the only functional imaging methodologies that allow to evaluate, in vivo in human, specific binding proteins such as receptors, transporters or enzymes. PET and SPECT have already proved to be unique tools to follow, in the living human brain, the kinetics of the interaction of a radiolabelled ligand with its receptors. However, these imaging techniques measure the radioligand concentration in regions of interest (ROIs) as a function of time but they do not allow the direct measurement of the binding parameters, i.e. receptor concentration and radioligand affinity. To estimate these physiological parameters a mathematical model must be designed to simulate the kinetics of the radioligand. The modelling of the data obtained using such equilibrium or dynamic models allow to extract from the kinetic data these physiological parameters. PET and SPECT imaging methodologies have then opened a new era in brain biochemistry and have already important applicants in brain physiopathology, clinical pharmacology and drug development.

Parametric images of benzodiazepine receptor concentration using a partial-saturation injection

The in vivo quantification of the benzodiazepine receptor concentration in human brain using positron emission tomography (PET) and 11C-flumazenil (11C-FMZ), is usually based on a three-compartment model and on PET curves measured in a small number of large regions of interest; however, it should be interesting to estimate the receptor concentration for each pixel and to build quantified images of the receptor concentration. The main advantage is to allow screening of the receptor site localization and visual observation of the possible abnormalities. Up to now, all the methods described include complex experimental protocols, difficult to use in routine examinations. In this paper, we propose the partial-saturation approach to obtain parametric images of benzodiazepine receptor concentration and FMZ affinity. It consists of a single FMZ injection with a low specific activity, followed by Scatchard analysis. Like other parametric imaging methods, this partial-saturation approach can lead to a small percentage (< 1%) of unrealistic values in receptor-poor regions; however, it is the only method that allows receptor concentration and affinity images to be obtained from a single-injection 40-min experiment without blood sampling. We also propose a second method in which the receptor concentration map is directly deduced from the PET image acquired 5 to 10 min after a partial-saturation injection. This method assumes a known and constant FMZ affinity value but requires only very simple corrections of this PET image. It is robust (negative values are never found) and quite simple to use in routine examination of patients (no blood sampling, single injection, only 10-min experiment).

Error analysis on parameter estimates in the ligand-receptor model: application to parameter imaging using PET data

Positron emission tomography and compartmental models allow the in vivo analysis of radioligand binding to receptor sites in the human brain. Benzodiazepine receptor binding was studied using a three-compartmental model and [11C]flumazenil. Four and five parameters were estimated from a single kinetic curve obtained with a multi-injection protocol, and parametric maps of receptor density and of the individual kinetic parameters were created with four-pixel sampling of the experimental images. The coefficient of variation on each estimated model parameter was calculated using the diagonal elements of the covariance matrix. However, these estimates are valid only under some statistical hypotheses which are not always verified with PET data. Thus, in order to verify the validity of the coefficient of variation of each parameter calculated with the covariance matrix, these results have been compared with the more rigorous statistical results provided by a Monte Carlo simulation. The study showed a negligible difference between the results obtained by the two methods for a low noise level in time-concentration curves encountered using large ROIs. However, this bias becomes less negligible when the noise level is high and some estimations of the coefficients of variation were unacceptable (> 100%) with the five-parameter model. Such difficulties did not occur with the four-parameter model which led to parametric images with good quality and acceptable estimates of coefficients of variation (less than 20% in about 75% of the ROIs).

Quantitation of benzodiazepine receptors in human brain using the partial saturation method

The in vivo quantification of the benzodiazepine receptor concentration in humans using PET and flumazenil (FMZ) is usually based on Scatchard analysis when the goal is to avoid blood sampling. The experimental protocols, however, include several (at least two) experiments with various specific activities in the same subject to obtain a range of bound ligand concentrations.

METHODS

We propose the partial saturation method, which is based on a natural decrease in bound ligand concentration after an FMZ injection with an average dose between a tracer dose and a saturation dose. An adequate range of bound ligand concentrations can thus be obtained from a single experiment. The free ligand concentration is estimated from the PET measurement in the pons after correction for the effect of the small receptor site concentration in this reference region.

RESULTS

The receptor concentration and affinity estimates obtained with this approach in six regions of interest agree with previously published values obtained by using more complex approaches. Receptor concentration appears to be insensitive to the uncertainties with regard to the receptor site concentration in the pons.

CONCLUSION

The partial saturation protocol can be used to estimate both the benzodiazepine receptor concentration and the FMZ affinity in routine examinations in adults (or even in children) using a single 40-min experiment without blood sampling.

Concept of reaction volume in the in vivo ligand-receptor model

In vivo quantification of receptor concentration and ligand affinity using data obtained with PET is based on the compartmental analysis of ligand-receptor interactions. There is, however, an inconsistency between the assumed homogeneity of the ligand concentration in each compartment, a basic hypothesis of the compartmental analysis, and the obvious heterogeneity of the tissue. Our goal was to study the effects of the free ligand concentration heterogeneity on the parameters describing in vivo binding reaction and to introduce the concept of reaction volume, VR, to account for that heterogeneity.

METHODS

The reaction volume is defined as the volume in which the free ligand mass present in 1 ml of tissue would have uniformly distributed with the same concentration as that in the vicinity of the receptor sites. The consequence of the heterogeneity of the free ligand concentration is that the equilibrium dissociation rate constant estimated from PET data corresponds to KdVR and not to Kd alone (defined by the ratio of the dissociation over the association rate constants). As a consequence, it is proposed to estimate the reaction volume as the ratio between the equilibrium dissociation constants obtained from in vivo and in vitro data (KdVR and Kd, respectively).

RESULTS

We used data obtained from studies performed with eight different molecules and found a correlation between the reaction volume and the molecule lipophilicity. This correlation can be used as a method to estimate the order of magnitude of VR from the lipophilicity which is easily accessible experimentally.

CONCLUSION

Reaction volume is an important parameter in in vivo ligand-receptor interaction modeling.

Parameter and index images of benzodiazepine receptor concentration in the brain

In vivo studies of ligand-receptor interactions with PET data are based on different approaches that provide either quantitative results (receptor density and affinity) or indices that are assumed to be correlated with the receptor concentration. The aims of this study are to obtain parametric images of benzodiazepine receptor concentration and of flumazenil affinity and to study the validity of two receptor concentration indexes.

METHODS

A three-compartment ligand-receptor model, [11C]flumazenil, and experimental data obtained using a three-injection protocol in human volunteers were used to acquire parametric images. The delayed activity method and the apparent distribution volume (estimated using a two-compartment model) were also tested and their results compared with those of the multi-injection approach.

RESULTS

Parametric images of receptor density, affinity and all kinetic parameters were obtained with acceptable variation coefficients. A correlation between receptor density and apparent affinity was found (r = 0.83; p < 0.0005). The correlation between receptor concentration and apparent distribution volume (estimated with three- and two-compartment models, respectively) was accessed using both a linear (the usual hypothesis) and a nonlinear correlation derived from the relationship between the receptor density and the affinity.

CONCLUSION

In spite of the complexity of this protocol (three injections, a 2-hr experiment, blood sampling and a metabolite study), we showed that the multi-injection approach is suitable for parametric brain imaging. By using this approach as a reference, we deduced that the distribution volume and delayed activity images are valid methods in the usual range of the benzodiazepine receptor concentrations found in the human brain.

Quantification of benzodiazepine receptors in human brain using PET, [11C]flumazenil, and a single-experiment protocol

A kinetic method using a multiinjection protocol, positron emission tomography (PET), and [11C]flumazenil as a specific ligand was used to study in vivo the flumazenil-benzodiazepine receptor interactions in the human brain. The model structure is composed of three compartments (plasma, free, and bound ligand) and five parameters (including the benzodiazepine receptor concentration). The arterial plasma concentration, after correction for metabolites, was used as the input function. The experimental protocol, which consisted of three injections of labeled and/or unlabeled ligand, allowed the evaluation of the five model parameters in various brain regions from a single experiment. In particular, the concentration of receptor sites available for binding (B'max) and the equilibrium dissociation constant (KDVR, VR being the volume of reaction) were estimated in five brain regions, including the pons, in which these parameters are identified for the first time (B'max = 4.7 +/- 1.7 pmol/ml and KDVR = 4.4 +/- 1.3 pmol/ml). Due to the large range of measured receptor concentrations, a linear correlation between B'max and KDVR was pointed out (r = 0.88, p < 0.0005) and was interpreted as a linear relationship between B'max and VR, the parameter KD being assumed constant. This result and its concordance with the published data are discussed. Simulation of the usual two-experiment Scatchard analysis, using the pons as a reference region, showed that the bias on the receptor concentration estimates introduced by this method is significant (from 20 to 40%) but can be corrected using an estimate of the receptor concentration in the pons. Furthermore, we propose a new experimental protocol, based on a Scatchard analysis of the PET data obtained with a partial-saturation experiment. This single-injection protocol is entirely noninvasive, and thus the estimation of the benzodiazepine receptor concentration and of the flumazenil affinity is now possible in human patients using a single 1-h experiment without blood sampling.

Canine myocardial beta-adrenergic, muscarinic receptor densities after denervation: a PET study

In an effort to better understand cardiac neurotransmission, PET was serially used in dogs to assess changes in ventricular muscarinic (MR) and beta-adrenergic receptor (beta-AR) densities following chemical or surgical denervation.

METHODS

Beta-adrenergic and MR receptor concentrations were studied in beagle dogs nine days after chemical sympathectomy (using the neurotoxin 6-hydroxydopamine) or 3-7 wk and 23-28 wk after surgical intrapericardial denervation.

RESULTS

In control dogs (n = 13), global beta-AR and MR concentrations were 32 +/- 4 and 62.2 +/- 10.4 pmole/ml tissue, respectively. Nine days after 6-hydroxytk; 1opamine (n = 8), hemodynamic tests and MIBG scintigraphy demonstrated the destruction of cardiac sympathetic innervation. Beta-adrenergic density increased by 190% (p < 0.001) while MR density remained unchanged. Three to 7 wk after surgery (n = 5), hemodynamic tests and MIBG scintigraphy demonstrated both parasympathetic and sympathetic denervations. Beta-adrenergic density was increased by 219% while MR concentration remained unchanged. Twenty-three to 28 wk after surgery, atrial innervation was restored (hemodynamic tests) while ventricular sympathetic innervation was not (MIBG scintigraphy). Beta-adrenergic density remained high.

CONCLUSION

The present study demonstrates the ability of PET to serially assess myocardial receptor concentrations. The absence of change in MR density and the prolonged up-regulation of beta-AR following heart denervation are the main findings of the present study.

In vivo quantification of myocardial muscarinic receptors in heart transplant patients

BACKGROUND

Decreased myocardial adenylate cyclase activity in response to guanine nucleotide stimulation has been recently demonstrated in denervated myocardium of transplant patients, suggesting that changes in left ventricular muscarinic receptors may occur.

METHODS AND RESULTS

The concentration and affinity constants of myocardial muscarinic receptors were determined by positron emission tomography with 11C-labeled methylquinuclidinyl benzilate (MQNB), a specific hydrophilic antagonist, in six transplant patients 4.7 +/- 2.3 months after surgery and in six normal subjects. Patients had no sign of cardiac rejection at endomyocardial biopsy. After intravenous injections of MQNB, time-activity curves were obtained over different regions of interest and were fitted to a nonlinear mathematical model. No difference in the concentration of muscarinic receptors was found in transplant patients compared with control subjects: 24 +/- 4 versus 26 +/- 7 pmol/mL tissue, respectively (P = NS). The association rate constant k+1, the dissociation rate constant k-1, and thus the equilibrium-dissociation constant Kd were the same in transplant patients compared with control subjects.

CONCLUSIONS

Despite known decreased GTP-stimulated adenylate cyclase activity in transplant patients, the density and affinity constants of myocardial muscarinic receptors are not altered. This suggests abnormalities of the signal-transduction function, such as a change in the guanine nucleotide binding proteins.

Quantification of myocardial muscarinic receptors with PET in humans

The potential for noninvasive quantification of myocardial muscarinic receptors using PET data, a mathematical model, multi-injection protocols and 11C-labeled methylquinuclidinyl benzilate (MQNB) as a radioligand was previously demonstrated in dogs. The present study examines the possibility of optimizing the experimental protocol to make this approach suitable for human studies. For six normal subjects, the protocol included three injections: a tracer injection, followed 30 min later by an injection of an excess of unlabeled MQNB (displacement) and then 30 min later by a simultaneous injection of unlabeled and labeled MQNB (coinjection). The model input function was estimated from the PET data corresponding to the left ventricular cavity. This protocol enables a separate evaluation of all parameters of a ligand-receptor model which includes three compartments and seven parameters. The complexity of this three-injection protocol, however, appears to be inconvenient for clinical use. A simplified two-injection protocol (tracer injection and coinjection) was evaluated in five other normal subjects and the results were compared to those obtained with the three-injection protocol. In regions of interest over the left ventricle, the mean value of the receptor concentration B'max and the equilibrium dissociation constant Kd were 26 +/- 7 pmole/ml tissue and 2.0 +/- 0.5 pmole/ml tissue, respectively. The possible existence of nonspecific binding was studied in two subjects using a double-displacement protocol. The corresponding rate constant was found to be very low (0.03 min-1).

Modeling analysis of [11C]flumazenil kinetics studied by PET: application to a critical study of the equilibrium approaches

The multi-injection modeling approach was used for the in vivo quantitation of benzodiazepine receptors in baboon brain using positron emission tomography (PET) and [11C]flumazenil (RO 15-1788) as a specific ligand. The model included three compartments (plasma, free, and bound ligand) and five parameters (including the benzodiazepine receptor concentration). The plasma concentration after correction for the metabolites was used as the input function. The experimental protocol consisted of four injections of labeled and/or unlabeled ligand. This protocol allows the evaluation, from a single experiment, of the five model parameters in various regions of interest. For example, in the temporal cortex, the concentration of receptor sites available for binding (B'max) and the equilibrium dissociation constant (Kd) were estimated to be 70 +/- 15 pmol/ml and 15.8 +/- 2.2 nM, respectively. The validity of the equilibrium approach, which is the most often used quantitation method, has been studied from simulated data calculated using these model parameters. The equilibrium approaches consist of reproducing in PET studies the experimental conditions that permit the use of the usual in vitro methods such as Scatchard analysis. These approaches are often open to criticism because of the difficulty of defining the notion of equilibrium in in vivo studies. However, it appears that the basic relation of Scatchard analysis is valid over a broader range of conditions than those normally used, such as the requirement of a constant bound/free ratio. Simulations showed that the values of the receptor concentration (B'max) and the equilibrium dissociation constant (Kd) found using Scatchard analysis are always underestimated. These simulations also suggest an explanation concerning the dependency of B'max and Kd on the time point employed for the Scatchard analysis, a phenomenon found by several authors. To conclude, we propose new protocols that allow the estimation of the B'max and Kd parameters using a Scatchard analysis but based on a protocol including only one or two injections. These protocols being entirely noninvasive, it thus becomes possible to investigate possible changes in receptor density and/or affinity in patients.

Positron emission tomography with 11C CGP-12177 to assess beta-adrenergic receptor concentration in idiopathic dilated cardiomyopathy

BACKGROUND

Positron emission tomography (PET) with 11C-labeled CGP-12177 (CGP) has been shown to have the potential to noninvasively measure beta-adrenergic receptor concentration in dog heart. The present study was undertaken to evaluate the clinical value of this technique.

METHODS AND RESULTS

Eight normal subjects and 10 patients with heart failure related to an idiopathic cardiomyopathy were studied. Estimation of beta-receptor concentration was based on a graphic method applied on myocardial PET time-concentration curves obtained after an intravenous injection of 11C-CGP followed 30 minutes later by a coinjection of labeled and unlabeled CGP. The clinical tolerance of these injections was good. Left ventricular concentration of beta-receptors was decreased in patients compared with controls (3.12 +/- 0.51 versus 6.60 +/- 1.18 pmol/mL, respectively; p < 0.001). This 53% decrease agrees with previous in vitro data. In eight of the 10 patients, the beta-receptor concentration obtained from PET was compared with the beta-receptor density determined on left ventricular endomyocardial biopsy samples by in vitro binding technique using 3H-CGP-12177. Results obtained with both techniques were correlated (r = 0.79, p = 0.019). Moreover, decreased beta-receptor concentration correlated with the beta-contractile responsiveness to intracoronary dobutamine infusion (r = 0.83, p = 0.003), indicating a direct link between changes in the receptor number and its biological function.

CONCLUSIONS

PET appears to be a safe and reliable method of assessing in vivo changes in the number of left ventricular beta-adrenergic receptor sites of patients with idiopathic cardiomyopathy.

Kinetic analysis of central [76Br]bromolisuride binding to dopamine D2 receptors studied by PET

The in vivo kinetic analysis of dopamine D2 receptors was obtained in baboon brain using positron emission tomography (PET) and [76Br]bromolisuride [( 76Br]BLIS) as radioligand. An injection of a trace amount of [76Br]BLIS was followed 3 h later by an injection of a mixture of [76Br]BLIS and BLIS in the same syringe (coinjection experiment). A third injection performed at 6 h was either an excess of unlabeled ligand (displacement experiment) or a second coinjection. This protocol allowed us to evaluate in the striatum of each animal and after a single experiment the quantity of available receptors (B'max) and the kinetic parameters including the association and dissociation rate constants (k + 1VR and k-1, respectively, where VR is the volume of reaction). The cerebellum data were fitted using a model without specific binding. All the parameters were estimated using nonlinear mathematical models of the ligand-receptor interactions including or not including nonspecific binding. The plasma time-concentration curve was used as an input function after correction for the metabolites. An estimate of standard errors was obtained for each PET study and for each identified parameter using the covariance matrix. The average values of B'max and KdVR were 73 +/- 11 pmol/ml tissue and 1.9 +/- 0.9 pmol/ml, respectively. The nonspecific binding was identifiable in the experiment where the last injection corresponded to a second coinjection. We found that approximately 6% of the striatal binding was nonspecific after a tracer injection of [76Br]BLIS. The nonspecific binding appeared to be reversible in the striatum but irreversible in the cerebellum.

Cardiac beta-adrenergic receptor density measured in vivo using PET, CGP 12177, and a new graphical method

The in vivo quantification of myocardial beta-adrenergic receptor has been obtained in five closed-chest dogs using positron emission tomography (PET). The ligand was racemic (+/-)[11C] CGP 12177, a very potent hydrophilic antagonist of the beta-adrenergic receptor. A kinetic method appeared unsuitable because of the presence of metabolites which made the input function difficult to measure and also inaccurate. Therefore, a graphical method, based on a particular protocol, was proposed. The animals were injected with a trace amount of (+/-)[11C]CGP 12177, which was followed 40 min later by a second injection of radioligand with a low-specific activity. An additional injection of an excess of unlabeled CGP 12177 was administered after 90 min and allowed for the estimation of the dissociation rate constant. The main advantage of this graphical approach is that the results are obtained without having to measure the input function and therefore without estimating the metabolites. The average value of Bmax was 31 +/- 4 pmole/ml of tissue and the dissociation constant was 0.014 +/- 0.002 min-1.

Noninvasive quantification of muscarinic receptors in vivo with positron emission tomography in the dog heart

The in vivo quantification of myocardial muscarinic receptors has been obtained in six closed-chest dogs by using positron emission tomography. The dogs were injected with a trace amount of 11C-labeled methylquinuclidinyl benzilate (MQNB), a nonmetabolized antagonist of the muscarinic receptor. This was followed 30 minutes later by an injection of an excess of unlabeled MQNB (displacement experiment). Two additional injections of unlabeled MQNB with [11C]MQNB (coinjection experiment) and without [11C]MQNB (second displacement experiment) were administered after 70 and 120 minutes, respectively. This protocol allowed a separate evaluation of the quantity of available receptors (B'max) as well as the association and dissociation rate constants (k+1 and k-1) in each dog. The parameters were calculated by using a nonlinear mathematical model in regions of interest over the left ventricle and the interventricular septum. The average value of B'max was 42 +/- 11 pmol/ml tissue, the rate constants k+1, k-1, and Kd were 0.6 +/- 0.1 ml.pmol-1.min-1, 0.27 +/- 0.03 ml.pmol-1.min-1, and 0.49 +/- 0.14 pmol.ml-1, respectively, taking into account the MQNB reaction volume estimated to 0.15 ml/ml tissue. Although [11C]MQNB binding would appear irreversible, our findings indicate that the association of the antagonist is very rapid and that the dissociation is far from negligible. The dissociated ligand, however, has a high probability of rebinding to a free receptor site instead of escaping into the microcirculation. We deduce that the positron emission tomographic images obtained after injecting a trace amount of [11C]MQNB are more representative of blood flow than of receptor density or affinity. We also suggest a simplified protocol consisting of a tracer injection of [11C]MQNB and a second injection of an excess of cold MQNB, which is sufficient to measure B'max and Kd in humans.

Identifiability analysis and parameter identification of an in vivo ligand-receptor model from PET data

Identifiability problem is a very important topic in the framework of model justification and not accounting for it during the modeling procedure can lead to meaningless results. While studying the receptor-ligand model parameter estimation from dynamic positron emission tomography data, each of the three possible conclusions to the identifiability problem (i.e., unidentifiable model, multiple solutions, or unique solution) are reached depending on the experimental protocol used. The identification of the model parameters from data obtained with a single tracer injection leads to disappointing numerical results since most of the parameters have to be considered as unidentifiable. A protocol including two injections, a first injection of the labeled ligand and a second injection of the cold ligand (displacement experiment) leads to two very different numerical solutions, which is surprising since such multiplicity of solutions was not indicated by a preliminary theoretical identifiability study. We show that a three-injections protocol, including both a displacement and coinjection experiment, allows to determine which of these two solutions is biologically valid.

Experimental design optimisation: theory and application to estimation of receptor model parameters using dynamic positron emission tomography

The general framework and various criteria for experimental design optimisation are presented. The methodology is applied to the estimation of receptor-ligand reaction model parameters with dynamic positron emission tomography data. The possibility of improving parameter estimation using a new experimental design combining an injection of the beta+-labelled ligand and an injection of the cold ligand is investigated. Numerical simulations predict a remarkable improvement in the accuracy of the parameter estimates with this new experimental design and particularly the possibility of separate estimations of the association constant (k+1) and of the receptor density (B'max) in a single experiment. Simulation predictions are validated using experimental PET data in which parameter uncertainties are reduced by factors ranging from 17 to 1000.

[Pulsed and continuous Doppler in qualitative and quantitative diagnosis of mitral insufficiency]

The aim of this study was to assess the diagnostic value of pulsed and continuous wave Doppler in mitral regurgitation. One hundred and twenty-one patients (64 women and 57 men aged 13 to 76 years, average 54 years) investigated for mitral regurgitation or ischaemic heart disease underwent left ventricular angiography and continuous wave and pulsed Doppler echocardiography. In addition to clinical examination, they also underwent M mode, 2D echocardiography and phonocardiography. They were divided into two groups according to the presence or absence of mitral regurgitation on angiography, chosen as the reference method. Group I comprised 51 patients with angiographic regurgitation, and Group II 70 patients without mitral regurgitation. The sensitivity of the Doppler examination was 98%. Of the 51 patients in Group I there was only one false negative in a patient with doubtful angiographic regurgitation in the context of an endocardial cushion defect. In comparison, the sensitivity of clinical examination and phonocardiography were 74.5% and 80% respectively; 13 cases of mitral regurgitation on angiography and Doppler echocardiography had no auscultatory signs. The specificity of the Doppler examination was 92.8%; 5 of the 70 patients in Group II had unquestionable systolic turbulence in the left atrium and 2D echocardiography showed the possible mechanism of these valvular leaks in 3 cases: 1 bivalvular prolapse, 1 rheumatic valvular thickening and 1 papillary muscle dysfunction. We interpret these 5 cases as being true mitral regurgitation but intermittent or too slight to be visible on angiography. The positive predictive value of systolic turbulence in the left was 90.9% and the negative predictive value was 98.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

[New criteria for a positive exercise test]

In this paper, the author reviews the recently developed criteria for a positive stress test. The "nearly ischaemic" appearance in which the ST segment is depressed by greater than or equal to 1.5 mm and slowly rises (slope of less than or equal to 1 millivolt/second) to a value which is predictive in symptomatic patients, of 70 p. cent in favour of coronary artery disease and 40 p. cent in favour of multiple vessel disease. The increased amplitude of the R wave in V5 immediately after effort is more a sign of left ventricular dysfunction than of coronary artery disease. The decreased amplitude of the Q wave in V5 and V6 at maximal effort is a very sensitive (82%) and very specific (88%) sign of left coronary or anterior interventricular artery disease. Similarly, inversion of the U wave in V5 on effort is a very sensitive and specific (97%) sign of left coronary or anterior interventricular artery disease. The major value of these new criteria is to help recognise the false negative results of the stress test, which can be as high as 50% of cases in patients with one vessel disease.

Brain uptake of 11C-methionine in phenylketonuria

The brain uptake of 11C-methionine was studied in 26 children with classical phenylketonuria; one adult was used as a control. Labelled methionine uptake in brain was first measured during a low phenylalanine diet and again one week later after a load of phenylalanine. Ten children aged 1 to 30 months were studied twice at intervals of several months. In children having a phenylalaninemia less than or equal to 0.3 mumoles . ml-1, a decrease in methionine brain uptake was observed with increasing age, with the largest change occurring during the first year of life. After the phenylalanine load, a mean increase in phenylalaninemia by a factor of ten was accompanied by a mean decrease in brain methionine uptake by a factor of two while blood methionine remained unchanged. Brain activity curves increased with time for children younger than one year and having phenylalaninemia less than 0.6 mumoles . ml-1. After the age of 2 most patients had a decreasing curve regardless of the blood phenylalanine level. This study indicates that 11C-methionine brain uptake may be taken as an index of blood barrier permeability to essential amino acids, and of brain maturation. The results obtained suggest that an increase in phenylalaninemia to levels greater than 0.6 mumole . ml-1 induces a modification in brain uptake of amino acids, primarily during the first two years of life.