A comprehensive compartmental model of blood glucose regulation for healthy and type 2 diabetic subjects.

We have expanded a former compartmental model of blood glucose regulation for healthy and type 2 diabetic subjects. The former model was a detailed physiological model which considered the interactions of three substances, glucose, insulin and glucagon on regulating the blood sugar. The main drawback of the former model was its restriction on the route of glucose entrance to the body which was limited to the intravenous glucose injection. To handle the oral glucose intake, we have added a model of glucose absorption in the gastrointestinal tract to the former model to address the resultant variations of blood glucose concentrations following an oral glucose intake. Another model representing the incretins production in the gastrointestinal tract along with their hormonal effects on boosting pancreatic insulin production is also added to the former model. We have used two sets of clinical data obtained during oral glucose tolerance test and isoglycemic intravenous glucose infusion test from both type 2 diabetic and healthy subjects to estimate the model parameters and to validate the model results. The estimation of model parameters is accomplished through solving a nonlinear optimization problem. The results show acceptable precision of the estimated model parameters and demonstrate the capability of the model in accurate prediction of the body response during the clinical studies.

Glucose-based optimization of CHO-cell perfusion cultures.

Perfusion cultures of CHO cells producing t-PA were performed using acoustic filter cell retention. A robust off-line glucose analysis and predictive control protocol was developed to maintain the process within approximately 0.5 mM of the glucose set point, without the need for a more fallible on-line sensor. Glucose usage (the difference between the inlet and reactor glucose concentrations) provided an easily measured indicator of overall medium utilization for mapping acceptable ranges of operation, including the edge of failure. Earlier onset of perfusion with a ramping glucose set point (1.5 mM/d) resulted in improved growth and consistency during the perfusion culture start-up. At steady state, the t-PA concentration variability increased gradually with increasing glucose usage up to approximately 22 mM, then up to 24 mM the variability increased threefold. Peak t-PA concentrations of over 90 mg/L were obtained by controlling at a glucose usage of approximately 24 mM, but these t-PA levels were not sustainable for more than 3 days. A consistent t-PA concentration of 40 mg/L was obtained at a glucose usage of 21.5 mM.

Increased t-PA yields using ultrafiltration of an inhibitory product from CHO fed-batch culture.

Fed-batch operation for the production of t-PA using Chinese Hamster Ovary (CHO) cells was optimized using serial and parallel experimentation. The feed, an isotonic concentrate, was improved to obtain 2- to 2.5-fold increases in integrated viable cell days versus batch. With a low glucose inoculum train, the viability index was further increased up to 4.5-fold. Hydrolysates were substituted for the amino acid portion of the concentrate with no significant change in fed-batch results. The concentrate addition rate was based on a constant 4 pmol/cell.day glucose uptake rate that maintained a relatively constant glucose concentration (approximately 3 mM). Increased viable cell indices did not lead to concomitant increases in t-PA concentrations compared to batch. The fed-batch concentrate and feeding strategy were shown to be effective in hybridoma culture, where a 4-fold increase in viable cell index yielded a 4-fold increase in antibody concentration. The half-life of t-PA decreased from 43 to 15 days with decreasing cell viability (from 92% to 71%), but this was not sufficient to explain the apparent t-PA threshold. Instead, the CHO results were explained by a reduction in t-PA production at higher extracellular t-PA concentrations that limited the fed-batch maximum at 35 mg/L for the cell line investigated. Analysis of both the total and t-PA mRNA levels revealed no response to increasing extracellular t-PA concentrations upon exogenous additions. Instead, intracellular t-PA levels were increased, revealing a possible secretory pathway limitation. A new reactor configuration was developed using an acoustic filter to retain the cells in the reactor while an ultrafiltration module stripped t-PA from the clarified medium before the permeate was returned to the reactor. By adding this harvesting step, the t-PA fed-batch production was increased over 2-fold, up to a yield of 80 mg/L.

Predictive control of hollow-fiber bioreactors for the production of monoclonal antibodies.

The selection of medium feed rates for perfusion bioreactors represents a challenge for process optimization, particularly in bioreactors that are sampled infrequently. When the present and immediate future of a bioprocess can be adequately described, predictive control can minimize deviations from set points in a manner that can maximize process consistency. Predictive control of perfusion hollow-fiber bioreactors was investigated in a series of hybridoma cell cultures that compared operator control to computer estimation of feed rates. Adaptive software routines were developed to estimate the current and predict the future glucose uptake and lactate production of the bioprocess at each sampling interval. The current and future glucose uptake rates were used to select the perfusion feed rate in a designed response to deviations from the set point values. The routines presented a graphical user interface through which the operator was able to view the up-to-date culture performance and assess the model description of the immediate future culture performance. In addition, fewer samples were taken in the computer-estimated cultures, reducing labor and analytical expense. The use of these predictive controller routines and the graphical user interface decreased the glucose and lactate concentration variances up to sevenfold, and antibody yields increased by 10% to 43%.

Evaluation of a long-range adaptive predictive controller for computerized drug delivery systems.

A closed-loop adaptive control system, based on the generalized predictive control law with a terminal matching condition, has been developed for computerized drug delivery. The control law is a minimization of the squares of prediction errors over a small future prediction horizon plus weighted square of the prediction error at steady-state. A control-relevant, long-range identification algorithm is used for on-line parameter estimation. Since the control and identification are mutually compatible, the system truly satisfies the approximate dual control criterion. The system has been applied to the control of mean arterial pressure (MAP) by automatic infusion of sodium nitroprusside in the presence of physical and physiological constraints. Experimental evaluation on six mongrel dogs, in an ethics-approved manner, included setpoint tracking and regulation of MAP in the presence of unpredictable disturbances. The system was found to be capable of inducing hypotension in an average of 2.44 +/- 0.31 min (mean +/- standard error of mean) after probing without any overshoots in mean arterial pressure. The nitroprusside infusion was also free of any ringing. When the subjects were not disturbed, 96.2% of mean arterial pressure remained within 5 mm Hg of the target pressure. A series of disturbances introduced in the presence and absence of closed-loop control affirms the robustness and effectiveness of this control system.