The usefulness of electrical stimulation for assessing pedicle screw placements.

The purpose of this study was to further establish the efficacy of pedicle screw stimulation as a monitoring technique to avoid nerve root injury during screw placement. The study population consisted of 662 patients in whom 3,409 pedicle screws were placed and tested by electrical stimulation. If stimulation resulted in a myogenic response at a stimulation intensity of 10 mA or less, the placement of the screw was inspected. Inspection was necessary for 3.9% of the screw placements in 15.4% of the study population. None of the patients in the study experienced any new postoperative neurologic deficits. These findings provide guidelines for the interpretation of stimulation data and support the use of this technique as an easy, inexpensive, and quick method to reliably assess screw placements and protecting neurological function.

Spinally elicited peripheral nerve responses are sensory rather than motor.

OBJECTIVES: Spinally elicited peripheral nerve responses, commonly called neurogenic motor evoked potentials (NMEPs), are widely used to monitor spinal cord motor function during surgery. However, numerous evidence suggests that these responses are primarily sensory rather than motor. The collision technique was utilized to address this issue. METHODS: Collision studies were performed in 7 patients during surgery. An ascending volley of sensory (AS) and motor activity (AM) was elicited by posterior tibial nerve stimulation at the popliteal fossa. After a short time delay, high cervical spinal stimulation produced a descending volley of sensory (DS) and motor (DM) activity. The AM volley ascended only to the anterior horn cells whereas the AS and DS volleys collided in the spinal cord. The inter-stimulus delays were varied so as to affect the degree of spinal cord collision. The DS and DM activity which remained after collision was recorded from the posterior tibial nerves at the ankle. RESULTS: Inter-stimulus delays of 18 ms or less resulted in no apparent peripheral descending volleys. These findings were consistent for all the patients studied. CONCLUSIONS: Spinally elicited peripheral nerve responses are primarily sensory rather than motor and are mediated by the same neural pathways as SEPs.

In vitro evaluation of percutaneous absorption of an acyclovir product using intact and tape-stripped human skin.

PURPOSE: To evaluate the use of a flow-through diffusion cell system to assess the absorption and penetration characteristics of drug (acyclovir) products. METHODS: In vitro studies were performed to assess the absorption/penetration of acyclovir using a flow-through diffusion cell system with human skin sections obtained from 19 healthy women following mammoplasty. The skin sections, 200-400 microm thick, were prepared using a dermatome. Acyclovir ointment (approximately 10 mg) spiked with (3)H-labelled acyclovir was applied onto the stratum corneum/epidermis side. The skin sections were continually perfused on the dermis side with sterilized culture medium, Buffered Hanks' Balanced Salt Solution, saturated with a CO(2)/O(2) (5/95%). RESULTS: After 24 hours, the percentages of acyclovir-derived radioactivity (based on dose applied) in different components were as follows: stratum corneum (SC), 0.20+0.28; viable skin (VS), 0.40+0.38; effluent fluid (EF), 0.25+0.53. A second set of experiments was performed using tape stripped (10X) skin sections. Levels of acyclovir-derived radioactivity were VS, 1.21+1.43 and EF, 2.65+2.61, which were threefold higher (p < 0.05) for VS and elevenfold higher (p < 0.05) for EF compared to the results obtained with the intact skin sections. CONCLUSIONS: The SC is the main barrier layer for the penetration of acyclovir through human skin. The use of the flow-through diffusion cell system provides an appropriate in vitro model to assess the absorption/penetration of acyclovir through human skin layers and therefore can potentially be used for dermal formulation characterization and development.

Drug dissolution into micellar solutions: development of a convective diffusion model and comparison to the film equilibrium model with application to surfactant-facilitated dissolution of carbamazepine.

The intrinsic dissolution rate of carbamazepine in solutions of sodium lauryl sulfate was measured to study the convective diffusion transport of drug-loaded micelles from a rotating disk. Alternative definitions for effective diffusivity and reaction factor are presented and compared with those commonly used for this type of transport problem. The conventional and alternative approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. For example, in this study it was observed that, above the cmc, a 2% w/v solution of sodium lauryl sulfate increased the dissolution rate approximately 6-fold and the solubility approximately 20-fold. This difference between the solubility and dissolution enhancement was attributed to the contribution to the total transport of both the enhanced solubility, a 20-fold increase, and the effective diffusivity of the drug-micelle complex, a 3-fold decrease, hence a net 6-fold increase in dissolution. The diffusivity of the drug-loaded micelle estimated from the dissolution data using the new definitions compared well with values determined by other methods (Dsm = 8.4 x 10(-7) cm2/s). On the basis of these results, the new definitions for the effective diffusivity and reaction factor offer a practical method for estimating micellar diffusion coefficients and predicting drug dissolution under the well-defined hydrodynamics of the rotating disk. It may also be possible to extend the application of these definitions to study the dissolution of water-insoluble drugs in other media, such as emulsions, to better understand drug dissolution under fed conditions in vivo.

Effects of humidity and temperature on in vitro dissolution of carbamazepine tablets.

The rate and extent of dissolution of various approved marketed carbamazepine (CBZ) tablets exposed to 33, 52, 75, and 97% relative humidities at both room temperature and 40 degrees C, and saturated water vapor at room temperature were compared to fresh unstressed tablets. The dissolution data indicate that exposure of CBZ tablets to high humidity and temperature can have a profound effect on tablet disintegration and dissolution. The dissolution rates of some batches of CBZ products exposed to 97% humidity at 40 degrees C or saturated water vapor at room temperature were drastically reduced in only 6-7 days.

In vitro/in vivo correlations in biopharmaceutics: scientific and regulatory implications.

This paper explains the regulatory and scientific reasons for the regulatory authorities employing dissolution as a key variable for regulatory approval of batch to batch bioequivalence assurance, site of manufacture change, formulation changes, and batch size scale-up for immediate release dosage forms. It also explains the scientific and regulatory reasons why either an in vivo/in vitro correlation using USP's Level 'A', 'B' or 'C', or a newly proposed 'mapping' approach will be required for allowing such changes for controlled-release dosage forms.

In vitro release of nitroglycerin from topical products by use of artificial membranes.

An in vitro testing method for measuring the release of nitroglycerin from topical drug products was evaluated. The method involved measuring the amount of nitroglycerin that diffused from ointments and patches through various synthetic membranes into receptor fluid contained in a modified Franz diffusion cell. Plots of the amount of nitroglycerin released against the square root of time for all 10 synthetic membranes were linear. Five membranes (group I) were found to release nitroglycerin at similar rates (difference not significant; p > 0.05). Use of the other five membranes (group II) in diffusion cells resulted in release rates significantly slower than those of group I membranes (p < 0.05). Rapid permeation of nitroglycerin from a solution through a polysulfone membrane demonstrated a lack of significant diffusion barrier properties of this membrane. Rates of release of nitroglycerin from commercially available ointments were found to be similar. A comparison of three commercial nitroglycerin patches revealed that these products released nitroglycerin at different rates in vitro.

High performance liquid chromatographic analysis of six conjugated and unconjugated estrogens in serum.

Six estrogens-estrone, equilin, estradiol, sodium estrone sulphate, sodium equilin sulphate and sodium 17-alpha-dihydroequilin sulphate in dog serum were successfully separated and quantified by a high performance liquid chromatographic method developed in our laboratories. The mobile phase was optimized by studying the effects of an organic modifier (acetonitrile) and an ion pairing reagent (tetrabutylammonium hydroxide). The serum sample work-up procedure was designed to recover both conjugated and unconjugated estrogens. The optimum method involved acetonitrile to precipitate serum proteins/peptides and extract estrogens. Residues were reconstituted in 50% acetonitrile in water for injection. The detection limits for these six estrogens via UV detection ranged from 0.5 to 5 ng on-column with a signal-to-noise ratio (S/N) of 10 for a 20 microL injection and via fluorescence 0.1 ng on-column for 17-beta-estradiol. Validation data are included for all six estrogens.

Influence of higher rates of agitation on release patterns of immediate-release drug products.

The dissolution procedure serves as a quality control test to assure batch-to-batch uniformity and bioequivalence of a product once the bioavailability of the product has been established. It can also be used to detect manufacturing and/or process variations that could reduce product bioavailability. Dissolution testing must be conducted at an appropriate agitation rate. Tests conducted at high agitation rates may lose the ability to differentiate between good and bad products. Although the effect of high agitation rates has been known for some time, several immediate-release drug products still have United States Pharmacopeia (USP) monograph dissolution procedures that require very high agitation rates. A systematic survey was conducted on marketed tablets of chloroquine phosphate, griseofulvin, hydroxychloroquine sulfate, isocarboxazide, primaquine phosphate, and sulfadiazine. Each of these products has a USP monograph requiring a dissolution test at a paddle speed of 100 rpm. To study the influence of agitation rate on the dissolution rate of these products, dissolution studies were conducted at paddle speeds of 50, 75, and 100 rpm with the USP apparatus 2 (paddle method). The dissolution rate increased with an increase in the agitation rate from 50 to 75 rpm. However, no significant increase in the dissolution rate was noted with an increase in the agitation rate from 75 to 100 rpm. The data support the position that the higher agitation rate of 100 rpm is not necessary for a quality control procedure or a compendial standard for the products tested.

Relationship between in vivo skin blanching and in vitro release rate for betamethasone valerate creams.

Betamethasone valerate creams from two firms were evaluated using the skin blanching procedure. In both studies, the same cream formulation exhibited significantly higher blanching compared to the other product. An in vitro release rate was determined for these betamethasone valerate cream products using a diffusion cell system, with a cellulose acetate membrane and a 60% ethanol:water receptor medium. The release rate (flux) of betamethasone valerate was higher for the higher blanching formulation and was statistically different from the other product. The integrity of the cellulose acetate membrane in 60% ethanol:water mixture was ascertained using hydrocortisone cream product. The in vitro drug release method, using a diffusion cell system and a synthetic membrane, can serve as a good quality control test method for topical creams.

A new method of dissolution in vitro, the "Bio-Dis" apparatus: comparison with the rotating bottle method and in vitro: in vivo correlations.

The aim was to study a new method of dissolution in vitro, the "Bio-Dis" apparatus, and to compare it with the classical rotating bottle method. Several theophylline controlled-release drug dosage forms were studied. Dissolution testing was performed in increasing pH in standard conditions and after treatment with peanut oil in order to simulate high fat meals and to correlate the in vitro percent dissolved with the in vivo results obtained. The in vivo study was carried out on three groups of healthy volunteers receiving each dosage form in a randomized order just before a high fat breakfast or in the fasting state. The in vitro percent dissolved obtained was compared with those published results obtained with rotating bottles. A linear relationship was established between these results. From the in vivo absorbed percentages calculated according to the Wagner-Nelson method, a linear relation was found between the in vivo percent absorbed and the in vitro percent dissolved in the different conditions. The relationships observed are similar for all the forms under both conditions. The "Bio-Dis" offers advantages over the rotating bottle method. The study reported allows this dissolution apparatus to be proposed as an alternative to the rotating bottle apparatus.

Analytical methods validation: bioavailability, bioequivalence and pharmacokinetic studies. Conference report.

This is a summary report of the conference on Analytical Methods Validation: Bioavailability, Bioequivalence and Pharmacokinetic Studies. The conference was held from December 3 to 5, 1990 in the Washington, DC area and was sponsored by the American Association of Pharmaceutical Scientists, US Food and Drug Administration, Federation International Pharmaceutique, Health Protection Branch (Canada) and Association of Official Analytical Chemists. The purpose of the report is to represent our assessment of the major agreements and issues discussed at the conference. The report is also intended to provide guiding principles for validation of analytical methods employed in bioavailability, bioequivalence and pharmacokinetic studies in man and animals. The objectives of the conference were: 1. To reach a consensus on what should be required in analytical methods validation and the procedures to establish validation; 2. To determine processes of application of the validation procedures in the bioavailability, bioequivalence and pharmacokinetic studies; 3. To develop a report on analytical methods validation (which may be referred to in developing future formal guidelines). Acceptable standards for documenting and validating analytical methods with regard to processes, parameters or data treatments were discussed because of their importance in assessment of pharmacokinetic, bioavailability and bioequivalence studies. Other topics which were considered essential in the conduct of pharmacokinetic studies or in establishing bioequivalency criteria, including measurement of drug metabolites and stereoselective determinations, were also deliberated.

Stimulus selectivity and state dependence of activity in inferior temporal cortex of infant monkeys.

Inferior temporal cortex is necessary for visual object recognition in adult primates but is less critical in infants. Nonetheless, in macaques as young as 6 weeks old, inferior temporal neurons showed adult-like visual response properties, including form selectivity and bilateral receptive fields, indicating that extended maturation and visual experience may not be necessary for adult-like encoding of complex objects. However, before the animals were 4 months old, visual responsiveness was found in inferior temporal cortex only in awake monkeys performing a behavioral task and not in anesthetized ones, suggesting that extraretinal factors profoundly influence function in "association" cortex in developing as well as mature animals.