Pharmacokinetics help optimizing mycophenolate mofetil dosing in kidney transplant patients

BACKGROUND

Mycophenolic acid (MPA), the active metabolite of mycophenolate mofetil (MMF), is now routinely used as immunosuppressant in solid organ transplantation in a fixed daily dose regimen (2 g/d) in association with cyclosporine (CsA) and steroids. However, no correlation has been shown between fixed MMF dose and clinical outcome.

METHODS

Here we examined the possibility of optimizing MMF dosing by drug pharmacokinetic monitoring in 46 stable kidney transplant recipients. MPA plasma concentration profiles were measured by a reverse-phase high-performance liquid chromatography method 6-9 months after transplantation and related with routine laboratory analysis tests. Since MPA is extensively bound to serum albumin and only the free fraction is pharmacologically active, in a subgroup of 23 patients free plasma MPA was also determined.

RESULTS

Despite a comparable MMF dose, a large interindividual variability in both MPA area under the curve (AUC) from 0 to 12 h (range 10.1-99.8 microg/mL. h) and in trough levels (range 0.24-7.04 microg/mL) was found. Patients with AUC >40 microg/mL. h showed a better (p<0.05) renal function than patients with lower AUC (creatinine clearance 85.7+/-23.2 versus 64.5+/-17.5 mL/min), despite no difference in CsA dose, CsA AUC and blood CsA trough level. The percentage of free plasma MPA but not total MPA correlated with the red blood cell and leukocyte count.

CONCLUSIONS

Therapeutic MMF drug monitoring might contribute to a better management of kidney transplant recipient with the goal of optimizing drug dosing and limiting the risk of MMF-related toxicity.

Motor imagery and visual event recognition

In order to investigate the influence of covert motor processes in the recognition of visual events, we compared the response times (RT) in two similar tasks, one involving a to-be-grasped object and the other involving a to-be-observed object. In one task, we asked right-handed subjects to tell whether an observed screwdriver presented in different orientations and rotating on its main axis was screwing or unscrewing (screwdriver task). In the other task the visual stimuli were precisely the same, but subjects had to think of the screwdriver as being the pivot pin of an imagined clock, turning its hands from the back (clock task). They had to tell whether the imagined clock hands were moving clockwise or counterclockwise. In the screwdriver task, a prominent right-left asymmetry consisting of higher RTs for stimulus orientations awkward for a right-hand grip was present, suggesting that subjects adopted a strategy based upon mentally simulating the grabbing of the screwdriver handle with the dominant hand. Consistent with the hypothesis that the crucial factor that triggers these motor imagery processes is the "graspability" of the relevant object in the scene, in the clock task the right-left asymmetry disappeared in most subjects, RTs mirroring the symmetry of the visual stimuli. These findings indicate that, when interpreting a scene involving a to-be-grasped object, a strategy based upon motor imagery (mental grasping), probably unfolding procedural knowledge, is activated. When the scene involves a to-be-observed object, the recognition task can be accomplished through other, possibly visual, strategies.

Recognizing the motion of a graspable object is guided by handedness

Subjects viewing a rotating screwdriver presented in a motion picture were asked whether it was screwing or unscrewing. A difference emerged in the performance of right- and left-handers. In right-handers, stimulus orientations particularly awkward for a right-hand grip determined higher response times, compared with visually equivalent but more comfortable orientations. When they were also requested to imagine their right hand grasping the screwdriver, similar results were obtained. By imagining the left hand, response times increased and the differences between awkward and comfortable orientations disappeared. In contrast, consistent with their manual dominance, left-handers response times were similar to those obtained when imagining the left but distinct from those obtained when imagining the right hand. We conclude that handedness may guide the interpretation of visual events involving manipulable objects.

Viewer- and object-centered mental explorations of an imagined environment are not equivalent

In this study we compared viewer-centered (VC) with object-centered (OC) mental exploration of an imagined clock drawn on the ground. An upper case F portrayed on a computer screen was to be imagined standing up in the center of the clock. In Expt. 1, an adjustment task was used to verify that the discrimination of clock directions rendered by this perspective drawing was quite accurate. Precision was not affected by the imaginary size of the clock. In Expt. 2, subjects either (1) indicated the clock location pointed by the F given their viewing position (VC condition), or (2) their location at the periphery of the clock given the location pointed by the F (OC condition). Response latencies were proportional to the explored imaginary distance and increased with the size of the imagined environment. We found an additional mean processing time of at least 2 s in the OC condition with respect to the VC condition. Results are interpreted within Kosslyn's (Kosslyn, S.M., A cognitive neuroscience of visual cognition: further developments. In: R.H. Logie and M. Denis (Eds.), Mental Images in Human Cognition, Elsevier Science Publishers, Amsterdam, 1991, pp. 351-381 [17]) framework of cognitive neuroscience.

Visual tuning to kinematics of biological motion: the role of eye movements

The visual system is particularly sensitive to the covariation between velocity and curvature that constraint biological motion. Previous work showed that, when this biological constraint is satisfied, simple elliptical motion of a dot looks constant, although its velocity is highly non-uniform. This paper addresses the hypothesis that such a dynamic illusion is dependent upon smooth pursuit eye movements. Subjects had to adjust the kinematics of a dot moving along elliptical trajectories until they perceived a constant velocity. Different pursuit and fixation conditions were tested. The research shows that the dynamic illusion is largely independent of eye movements, suggesting that the visual system has access to implicit knowledge of motor constraints regardless of the concurrent oculomotor commands.

Cerebral dominance and asynchrony between bimanual two-dimensional movements

The asynchrony of bimanual movements was investigated. Right- and left-handers traced simple geometrical patterns (ellipses) continuously with both hands. All combinations of the direction of rotation in each hand were executed at different rhythms. Geometrically, performances were largely independent of manual dominance. However, by comparing the passage times at homologous positions, the authors found that the dominant hand led the nondominant one by about 25 ms. The asynchrony was affected by neither movement type nor rhythm. The variability of the asynchrony varied along the trajectory, with well-defined maxima and minima. The variability profiles for movements that engaged homologous muscles differed markedly from those that engaged nonhomologous muscles. The authors discuss the hypothesis that bimanual periodic movements are timed by a lateralized functional module and asynchrony is due to the necessity of transmitting time-keeping information to the other hemisphere.

Cerebral dominance and asynchrony between bimanual two-dimensional movements

The asynchrony of bimanual movements was investigated. Right- and left-handers traced simple geometrical patterns (ellipses) continuously with both hands. All combinations of the direction of rotation in each hand were executed at different rhythms. Geometrically, performances were largely independent of manual dominance. However, by comparing the passage times at homologous positions, the authors found that the dominant hand led the nondominant one by about 25 ms. The asynchrony was affected by neither movement type nor rhythm. The variability of the asynchrony varied along the trajectory, with well-defined maxima and minima. The variability profiles for movements that engaged homologous muscles differed markedly from those that engaged nonhomologous muscles. The authors discuss the hypothesis that bimanual periodic movements are timed by a lateralized functional module and asynchrony is due to the necessity of transmitting time-keeping information to the other hemisphere.

Biological movements look uniform: evidence of motor-perceptual interactions

Six experiments demonstrate a visual dynamic illusion. Previous work has shown that in 2-dimensional (2D) drawing movements, tangential velocity and radius of curvature covary in a constrained manner. The velocity of point stimuli is perceived as uniform if and only if this biological constraint is satisfied. The illusion is conspicuous: The variations of velocity in the stimuli exceed 200%. Yet movements are perceived as uniform. Conversely, 2D stimuli moving at constant velocity are perceived as strongly nonuniform. The illusion is robust: Exposure to true constant velocity fails to suppress it. Results cannot be explained entirely by the kinetic depth effect. The illusion is evidence of a coupling between motor and perceptual processes: Even in the absence of any intention to perform a movement, certain properties of the motor system implicitly influence perceptual interpretation of the visual stimulus.

Biological movements look uniform: Evidence of motor-perceptual interactions

Six experiments demonstrate a visual dynamic illusion. Previous work has shown that in 2-dimensional (2D) drawing movements, tangential velocity and radius of curvature covary in a constrained manner. The velocity of point stimuli is perceived as uniform if and only if this biological constraint is satisfied. The illusion is conspicuous: The variations of velocity in the stimuli exceed 200%. Yet movements are perceived as uniform. Conversely, 2D stimuli moving at constant velocity are perceived as strongly nonuniform. The illusion is robust: Exposure to true constant velocity fails to suppress it. Results cannot be explained entirely by the kinetic depth effect. The illusion is evidence of a coupling between motor and perceptual processes: Even in the absence of any intention to perform a movement, certain properties of the motor system implicitly influence perceptual interpretation of the visual stimulus.

The effect of movement velocity on form perception: geometric illusions in dynamic displays

We investigated the effects of movement velocity on the perception of simple geometric trajectories. We show that when an ellipse is traced by the continuous displacement of a spot against an empty background, the subjective aspect ratio (R = vertical axis/horizontal axis) of the figure depends on the law of motion of the spot. If the tangential velocity of the spot is constant, very large and subject-specific biases emerge in the perception of the aspect ratio. If the tangential velocity of the spot is made equal to that of an elliptic motion with aspect ratio R less than 1, and resulting from the vectorial composition of two harmonic functions (Lissajous motion) there is a general trend to perceive the ellipse as being flatter than in reality. The effect, however, is not symmetric: when the velocity follows a Lissajous modulation with R greater than 1, highly significant biases are still present in most subjects, but no common trend emerges from the experimental population. The results are discussed in the context of recent findings on the relationship between form and kinematics in spontaneous human movements.