Identification of age-structured models: cell cycle phase transitions.

A methodology is developed that determines age-specific transition rates between cell cycle phases during balanced growth by utilizing age-structured population balance equations. Age-distributed models are the simplest way to account for varied behavior of individual cells. However, this simplicity is offset by difficulties in making observations of age distributions, so age-distributed models are difficult to fit to experimental data. Herein, the proposed methodology is implemented to identify an age-structured model for human leukemia cells (Jurkat) based only on measurements of the total number density after the addition of bromodeoxyuridine partitions the total cell population into two subpopulations. Each of the subpopulations will temporarily undergo a period of unbalanced growth, which provides sufficient information to extract age-dependent transition rates, while the total cell population remains in balanced growth. The stipulation of initial balanced growth permits the derivation of age densities based on only age-dependent transition rates. In fitting the experimental data, a flexible transition rate representation, utilizing a series of cubic spline nodes, finds a bimodal G(0)/G(1) transition age probability distribution best fits the experimental data. This resolution may be unnecessary as convex combinations of more restricted transition rates derived from normalized Gaussian, lognormal, or skewed lognormal transition-age probability distributions corroborate the spline predictions, but require fewer parameters. The fit of data with a single log normal distribution is somewhat inferior suggesting the bimodal result as more likely. Regardless of the choice of basis functions, this methodology can identify age distributions, age-specific transition rates, and transition-age distributions during balanced growth conditions.

Analysis of resonance chemotherapy in leukemia treatment via multi-staged population balance models.

An age-structured population balance model that explicitly models cell cycle phases is developed to investigate the effects of cell cycle specific (CCS) drugs. In particular, the benefits of timing CCS drug treatments in resonance chemotherapy are predicted and measured directly in vitro before evaluating likely in vivo scenarios. The phase transition rates are measured in vitro for the HL60 leukemia cell line and are used to predict the transient phase dynamics after exposure to the S phase specific drug, camptothecin. The phase oscillations predicted by the model are observed experimentally and the timing of a second camptothecin pulse is shown to significantly alter the overall treatment effectiveness. To explore the feasibility of designing resonance chemotherapeutic treatments to preferentially eliminate one cell type over another, Jurkat and HL60 leukemia cells are exposed to the same dual-pulse camptothecin treatment regimen. With the model framework validated for simplified cases, the model is used to extrapolate the effectiveness of resonance chemotherapy considering in vivo effects such as quiescence, drug metabolism, drug properties, and transport considerations that were not included in the in vitro experiments. While resonance chemotherapy is intuitive and looks promising in vitro, when in vivo considerations are included in the model, the phenomenon is dampened and the window of applicability becomes narrower.

Evaluation of test-weighing for the assessment of milk volume intake of formula-fed infants and its application to breast-fed infants.

Test-weighing (TW) was evaluated in formula-fed (FF) infants by comparison with direct measurement (DM) of formula intake during a 24 h period at 1, 2, 4 and 6 mo of age. Formula intakes estimated by TW ranged from 87 to 93% of those determined by DM over the 6-mo period. During the study period the number of feedings per day decreased but were not significantly different for FF and BF infants. Volume of intake per feeding by FF infants increased significantly during the 6-mo period whereas milk intakes of BF infants were not different at the four ages studied. These data indicated that when the number of daily feedings decreased with age, FF infants increased their volume of intake per feeding whereas BF infants did not adjust their intakes. Mean volume of milk intake at 4 and 6 mo, estimated by TW, was significantly greater in FF infants compared to breast-fed (BF) infants matched for age and size. Data from this study indicated clearly that when milk intakes of BF infants are compared to those of FF infants, both groups of infants should be test-weighed.

PIP: Test-weighing (TW) was evaluated in formula-fed (FF) infants from a University community in Indiana by comparison with direct measurement (DM) of formula intake during a 24 hour period at 1, 2, 4, and 6 months of age. Formula intakes estimated by TW ranged from 87 to 93% of those determined by DM over the 6-month period. During the study period the number of feedings per day decreased but were not significantly different for FF and BF infants. Volume of intake per feeding by FF infants increased significantly during the 6-month period whereas milk intakes of BF infants were not different at the 4 ages studied. These data indicate that when the number of daily feedings decreased with age, FF infants increased their volume of intake per feeding whereas BF infants did not adjust their intakes. Mean volume of milk intake at 4 and 6 months, estimated by TW, was significantly greater in FF infants compared to BF infants matched for age and size. Data from this study indicate clearly that when milk intakes of BF infants are compared to those of FF infants, both groups of infants should be test-weighed.

Effects of vitamin B6 intake on nutriture and growth of young infants.

Vitamin B6 is critical to normal development; however, the requirement for adequate nutriture of the human infant is based on limited experimental data. In this study vitamin B6 intakes of breast-fed (BF) and formula-fed (FF), healthy, term infants were related to levels of pyridoxal phosphate (PLP) in their plasma at 1, 2, 4, and 6 mo of age. Mothers of BF infants were supplemented with either 2.5 or 15.0 mg pyridoxine . hydrochloride (PN . HCl)/d. Growth was similar for FF and BF infants and was within normal ranges over the 6 mo period. Plasma PLP in cord blood was similar in BF and FF infants; however, at 1-5 d of age and at each subsequent age studied, levels of plasma PLP were significantly higher in FF infants than in BF. Lowest PLP values were for BF infants of mothers who received 2.5 mg PN . HCl/d. Mean plasma PLP decreased with age and was not correlated to vitamin B6 intakes except at 1 mo of age. At this age, vitamin B6 intake of BF infants whose mothers received 2.5 mg PN . HCl/d was only 0.1 mg B6/d. The consequences of this are uncertain; however, plasma PLP levels of the infants were low and reflected their intakes of vitamin B6.

Digital plethysmography for assessing erythrityl tetranitrate bioavailability.

The bioavailability or oral, sublingual, and chewable tablets or erythrityl tetranitrate (ETN) was evaluated in 15 normal men. In a randomized, complete crossover investigation with nitroglycerin and placebo controls, drug-induced changes in the diastolic amplitude response intensity were measured with a digital plethysmogram. Values for area under the response intensity curve (AUC), maximum response intensity (Imax), and time lapse from dosing to peak response (tmax) were obtained by computer processing and converted to intensity values and AUC segments for specific time intervals. Sublingual nitroglycerin induced a response (p less than 0.05) from placebo within the first hour. Although somewhat slower in reaching peak intensity, all forms of ETN induced significant responses over placebo (p less than 0.05) for 2 hr, with oral (swallowed) ETN different 6 hr. Our results indicate that all the ETN dosage forms were bioavailable, with the longest duration of effect by the oral form.

Respiratory distress associated with elective repeat cesarean section. A two-year experience in one medical community.

One hundred and forty-five elective repeat cesarean sections performed in one large medical community during a two-year period were studied. Ninety-nine of the patients had no prenatal test for determination of pulmonary or fetal maturity; 18 patients had an amniocentesis for L/S ratio. Only two infants were thought to be delivered prematurely, at 37 weeks gestational age. One of these infants had no respiratory distress. The other infant was delivered before the EDC by menstrual dates because a single ultrasound measurement performed one week before delivery suggested a "term fetus", and this infant had severe hyaline membrane disease. Six other infants had mild respiratory distress compatible with transient tachypnea or aspiration. In this medical community, one must question whether the risk of routine amniocentesis for L/S ratio before every elective repeat cesarean section might outweigh the potential benefits of the procedure.

Determination of serum bilirubin by skin reflectance: effect of pigmentation.

A noninvasive optical technique by which serum bilrubin can be estimated from skin spectral reflectance measurements has been further investigated. The original work on 30 healthy, full-term white infants and an independent study on 14 white and 30 black infants demonstrate that the method has potential not only for clinical use, but also for the study of the transport of bilrubin to, from and within the skin. The objectives of the present study are to evaluate the method on a larger sample population with special attention to natural skin pigmentation effects and the development of a physical model of the tissue to explain the relationship between serum bilrubin concentration and skin reflectance. Reflectance spectra (380-800 nm) and concurrent serum bilirubin measurements were taken on a sample population of 58 white and 45 full-term black infants (1-3 days of age). Multiple linear regression analysis, comprised of six wavelengths gave a correlation coefficient, r = 0.831 for the white infant group. For the black infant group, a five wavelength analysis provided r = 0.877 with the standard error of estimate being +/- 1.46 mg/100 ml for both groups. The model for establishing a physical basis for the relationship shows that a transformed, normalized Kubelka-Monk function xi (460, 510, 420) is linearly related to serum bilrubin concentration. This function is determined from the spectral reflectance values at three wavelengths, 420, 460, and 510 nm. The wavelength combination is such that effects due to hemoglobin and melanin pigments are minimized. Regression analysis showed that r = 0.778 and r = 0.865 for the white and black infant groups, respectively, with standard error of estimates being +/- 1.4 mg/100 ml for both groups. Routine determinations of total serum bilrubin by laboratory methods have standard errors of estimate ranging from +/- 1 to 1.5 mg/100 ml. Thus, the method herein described shows that the relationship between skin reflectance and serum bilrubin in full-term infants is close to the acceptable limits for clinical use. Furthermore, this work shows that skin pigmentation does not obscure this relationship.