Saccharomyces cerevisiae NRE1 and IRC24 Encode Paralogous Benzil Oxidoreductases

Irc24p is a benzil oxidoreductase encoded on chromosome IX of Saccharomyces cerevisiae . We identified a putative paralog, Nre1p, encoded 284 bp downstream. Both proteins are small, cytoplasmic, and are 52% identical (70% similar). PANTHER and PFAM analysis of the amino acid sequences and rigid pairwise structure alignment predicted a conserved active site and Rossmann folds in both, implicating NADH or NADPH as likely cofactors. We purified hexahistidine-tagged Irc24p and Nre1p. Both proteins catalyze the reduction of the diketone benzil with similar kinetics and a preference for NADPH. This is the first demonstration of in vitro function for Nre1p.

Students authoring molecular case studies as a partial course-based undergraduate research experience (CURE) for lab instruction

Understanding the relationship between protein structure and function is a core-learning goal in biochemistry. Students often struggle to visualize proteins as three-dimensional objects that interact with other molecules to affect its biochemical consequences. We describe here a partial course-based undergraduate research experiences that has students exploring protein structure and function hands-on while authoring a molecular case study intended for others to use.

Epstein-Barr viral microRNAs target caspase 3

The Epstein-Barr virus (EBV) is a ubiquitous herpesvirus that transforms B cells and causes several malignancies including Burkitt’s lymphoma. EBV differentially expresses at least 49 mature microRNAs (miRNAs) during latency in various infected epithelial and B cells. Recent high-throughput studies and functional assays have begun to reveal the function of the EBV miRNAs suggesting roles in latency, cell cycle control, and apoptosis. In particular, the central executioner of apoptosis, Caspase 3 (CASP3), was proposed as a target of select EBV miRNAs. However, whether CASP3 is truly a target of EBV miRNAs, and if so, which specific miRNAs target CASP3 is still under debate. Based on previously published high-throughput biochemical data and a bioinformatic analysis of the entire CASP3 3′-UTR, we identified 12 EBV miRNAs that have one or more seed binding sites in the CASP3 3′-UTR. We individually tested all 12 miRNAs for repression of CASP3 in luciferase reporter assays, and nine showed statistically significant (P < 0.001) repression of a full-length CASP3 reporter. Further, three EBV miRNAs, including BART22, exhibited repression of endogenous CASP3 protein. These data confirm that CASP3 is a direct target of specific EBV BART miRNAs.

Alternative capture of noncoding RNAs or protein-coding genes by herpesviruses to alter host T cell function

In marmoset T cells transformed by Herpesvirus saimiri (HVS), a viral U-rich noncoding (nc) RNA, HSUR 1, specifically mediates degradation of host microRNA-27 (miR-27). High-throughput sequencing of RNA after crosslinking immunoprecipitation (HITS-CLIP) identified mRNAs targeted by miR-27 as enriched in the T cell receptor (TCR) signaling pathway, including GRB2. Accordingly, transfection of miR-27 into human T cells attenuates TCR-induced activation of mitogen-activated protein kinases (MAPKs) and induction of CD69. MiR-27 also robustly regulates SEMA7A and IFN-γ, key modulators and effectors of T cell function. Knockdown or ectopic expression of HSUR 1 alters levels of these proteins in virally transformed cells. Two other T-lymphotropic γ-herpesviruses, AlHV-1 and OvHV-2, do not produce a noncoding RNA to downregulate miR-27 but instead encode homologs of miR-27 target genes. Thus, oncogenic γ-herpesviruses have evolved diverse strategies to converge on common targets in host T cells.

The "Observer Effect" in genome-wide surveys of protein-RNA interactions

Recent technological advances have spurred genome-wide studies that afford insights into ribonucleoprotein biology and transcript regulation on an unprecedented scale. Here we review techniques currently used to obtain genome-wide profiles of RNA-protein interactions in living cells. We highlight recent studies of the mRNA-bound proteome and address pitfalls inherent in such investigations.

Association of Argonaute proteins and microRNAs can occur after cell lysis

MicroRNA (miRNA) target identification is a challenging but important endeavor. Global analyses of the direct mRNA targets of miRNAs have relied heavily upon immunopurification techniques, wherein a core protein component of the miRNA-protein complex, Argonaute (Ago), is immunoprecipitated to isolate associated RNAs. This approach involves the assumption that the selected RNAs were bound to the Ago protein in vivo and that the methodology did not significantly perturb endogenous interactions or produce novel interaction artifacts. To test whether RNAs that coimmunoprecipitate with human Ago were bound in vivo or could associate post-cell lysis, we used an experimental approach that distinguishes between these two origins of interaction. We show that a transfected miRNA mimic, but not a plasmid-expressed miRNA, can interact with human Ago proteins post-lysis. Our results have important implications for the design of miRNP immunoprecipitation experiments.

An international dosimetry exchange for BNCT part II: computational dosimetry normalizations

The meaningful sharing and combining of clinical results from different centers in the world performing boron neutron capture therapy (BNCT) requires improved precision in dose specification between programs. To this end absorbed dose normalizations were performed for the European clinical centers at the Joint Research Centre of the European Commission, Petten (The Netherlands), Nuclear Research Institute, Rez (Czech Republic), VTT, Espoo (Finland), and Studsvik, Nyköping (Sweden). Each European group prepared a treatment plan calculation that was bench-marked against Massachusetts Institute of Technology (MIT) dosimetry performed in a large, water-filled phantom to uniformly evaluate dose specifications with an estimated precision of +/-2%-3%. These normalizations were compared with those derived from an earlier exchange between Brookhaven National Laboratory (BNL) and MIT in the USA. Neglecting the uncertainties related to biological weighting factors, large variations between calculated and measured dose are apparent that depend upon the 10B uptake in tissue. Assuming a boron concentration of 15 microg g(-1) in normal tissue, differences in the evaluated maximum dose to brain for the same nominal specification of 10 Gy(w) at the different facilities range between 7.6 and 13.2 Gy(w) in the trials using boronophenylalanine (BPA) as the boron delivery compound and between 8.9 and 11.1 Gy(w) in the two boron sulfhydryl (BSH) studies. Most notably, the value for the same specified dose of 10 Gy(w) determined at the different participating centers using BPA is significantly higher than at BNL by 32% (MIT), 43% (VTT), 49% (JRC), and 74% (Studsvik). Conversion of dose specification is now possible between all active participants and should be incorporated into future multi-center patient analyses.

Unifying dose specification between clinical BNCT centers in the Americas

A dosimetry intercomparison between the boron neutron capture therapy groups of the Massachusetts Institute of Technology (MIT) and the Comisión Nacional de Energía Atómica (CNEA), Argentina was performed to enable combined analyses of NCT patient data between the different centers. In-air and dose versus depth measurements in a rectangular water phantom were performed at the hyperthermal neutron beam facility of the RA-6 reactor, Bariloche. Calculated dose profiles from the CNEA treatment planning system NCTPlan that were calibrated against in-house measurements required normalizations of 1.0 (thermal neutrons), 1.13 (photons), and 0.74 (fast neutrons) to match the dosimetry of MIT.

Normalisation of prescribed dose in BNCT

Normalisation of prescribed dose in boron neutron capture therapy (BNCT) is needed to facilitate combining clinical data from different centres in the world to help expedite development of the modality. The approach being pursued within the BNCT community is based upon improving precision in the measurement and specification of absorbed dose. Beam characterisations using a common method are complete as are comparative dosimetry measurements between clinical centres in Europe and the USA. Results from treatment planning systems at these centres have been compared with measurements performed by MIT, and the scale factors determined are being confirmed with independent tests using measurements in an ellipsoidal water phantom. Dose normalisations have successfully been completed and applied to retrospectively analyse treatment plans from Brookhaven National Laboratory (1994-99) so that reported doses are consistently expressed with the trials performed during 1994-2003 at Harvard-MIT. Dose response relationships for adverse events and other endpoints can now be more accurately established.

An international dosimetry exchange for boron neutron capture therapy. Part I: Absorbed dose measurements

An international collaboration was organized to undertake a dosimetry exchange to enable the future combination of clinical data from different centers conducting neutron capture therapy trials. As a first step (Part I) the dosimetry group from the Americas, represented by MIT, visited the clinical centers at Studsvik (Sweden), VTT Espoo (Finland), and the Nuclear Research Institute (NRI) at Rez (Czech Republic). A combined VTT/NRI group reciprocated with a visit to MIT. Each participant performed a series of dosimetry measurements under equivalent irradiation conditions using methods appropriate to their clinical protocols. This entailed in-air measurements and dose versus depth measurements in a large water phantom. Thermal neutron flux as well as fast neutron and photon absorbed dose rates were measured. Satisfactory agreement in determining absorbed dose within the experimental uncertainties was obtained between the different groups although the measurement uncertainties are large, ranging between 3% and 30% depending upon the dose component and the depth of measurement. To improve the precision in the specification of absorbed dose amongst the participants, the individually measured dose components were normalized to the results from a single method. Assuming a boron concentration of 15 microg g(-1) that is typical of concentrations realized clinically with the boron delivery compound boronophenylalanine-fructose, systematic discrepancies in the specification of the total biologically weighted dose of up to 10% were apparent between the different groups. The results from these measurements will be used in future to normalize treatment plan calculations between the different clinical dosimetry protocols as Part II of this study.

RBE of the MIT epithermal neutron beam for crypt cell regeneration in mice

The RBE of the new MIT fission converter epithermal neutron capture therapy (NCT) beam has been determined using intestinal crypt regeneration in mice as the reference biological system. Female BALB/c mice were positioned separately at depths of 2.5 and 9.7 cm in a Lucite phantom where the measured total absorbed dose rates were 0.45 and 0.17 Gy/ min, respectively, and irradiated to the whole body with no boron present. The gamma-ray (low-LET) contributions to the total absorbed dose (low- + high-LET dose components) were 77% (2.5 cm) and 90% (9.7 cm), respectively. Control irradiations were performed with the same batch of animals using 6 MV photons at a dose rate of 0.83 Gy/min as the reference radiation. The data were consistent with there being a single RBE for each NCT beam relative to the reference 6 MV photon beam. Fitting the data according to the LQ model, the RBEs of the NCT beams were estimated as 1.50 +/- 0.04 and 1.03 +/- 0.03 at depths of 2.5 and 9.7 cm, respectively. An alternative parameterization of the LQ model considering the proportion of the high- and low-LET dose components yielded RBE values at a survival level corresponding to 20 crypts (16.7%) of 5.2 +/- 0.6 and 4.0 +/- 0.7 for the high-LET component (neutrons) at 2.5 and 9.7 cm, respectively. The two estimates are significantly different (P = 0.016). There was also some evidence to suggest that the shapes of the curves do differ somewhat for the different radiation sources. These discrepancies could be ascribed to differences in the mechanism of action, to dose-rate effects, or, more likely, to differential sampling of a more complex dose-response relationship.

Epithermal Neutron Beams for Clinical Studies of Boron Neutron Capture Therapy: A Dosimetric Comparison of Seven Beams

A comparison of seven epithermal neutron beams used in clinical studies of boron neutron capture therapy (BNCT) in Sweden (Studsvik), Finland (Espoo), Czech Republic (ReZ), The Netherlands (Petten) and the U.S. (Brookhaven and Cambridge) was performed to facilitate sharing of preclinical and clinical results. The physical performance of each beam was measured using a common dosimetry method under conditions pertinent to brain irradiations. Neutron fluence and absorbed dose measurements were performed with activation foils and paired ionization chambers on the central axis both in air and in an ellipsoidal water phantom. The overall quality of each beam was assessed by figures of merit determined from the total weighted dose profiles that assumed the presence of boron in tissue. The in-air specific beam contamination from both fast neutrons and gamma rays ranged between 8 and 65 x 10(-11) cGy(w) cm2 for the different beams and the epithermal neutron flux intensities available at the patient position differed by more than a factor of 20 from 0.2-4.3 x 10(9) n cm(-2) s(-1). Percentage depth dose profiles measured in-phantom for the individual photon, thermal and fast-neutron dose components differed only subtly in shape between facilities. Assuming uptake characteristics consistent with the currently used boronated phenylalanine, all the epithermal beams exhibit a useful penetration of 8 cm or greater that is sufficient to irradiate a lesion seated at the brain midline. The performance of the existing facilities will benefit from the introduction of advanced compounds through improved beam penetrability. This could increase by as much as 2 cm for the purest of beams, although the beam intensities generally need to be increased to between 2-5 x 10(9) n cm(-2) s(-1) to maintain manageable irradiation times. These data provide the first consistent measurement of beam performance at the different centers and will enable a preliminary normalization of the calculated patient dosimetry.

Preliminary treatment planning and dosimetry for a clinical trial of neutron capture therapy using a fission converter epithermal neutron beam

A Phase I/II clinical trial of neutron capture therapy (NCT) was conducted at Harvard-MIT using a fission converter epithermal neutron beam. This epithermal neutron beam has nearly ideal performance characteristics (high intensity and purity) and is well-suited for clinical use. Six glioblastoma multiforme (GBM) patients were treated with NCT by infusion of the tumor-selective amino acid boronophenylalanine-fructose (BPA-F) at a dose of 14.0 g/m(2) body surface area over 90 min followed by irradiation with epithermal neutrons. Treatments were planned using NCTPlan and an accelerated version of the Monte Carlo radiation transport code MCNP 4B. Treatments were delivered in two fractions with two or three fields. Field order was reversed between fractions to equalize the average blood boron concentration between fields. The initial dose in the dose escalation study was 7.0 RBEGy, prescribed as the mean dose to the whole brain volume. This prescription dose was increased by 10% to 7.7 RBEGy in the second cohort of patients. A pharmacokinetic model was used to predict the blood boron concentration for determination of the required beam monitor units with good accuracy; differences between prescribed and delivered doses were 1.5% or less. Estimates of average tumor doses ranged from 33.7 to 83.4 RBEGy (median 57.8 RBEGy), a substantial improvement over our previous trial where the median value of the average tumor dose was 25.8 RBEGy.

Progress with the NCT international dosimetry exchange

The international collaboration that was organized to undertake a dosimetry exchange for purposes of combining clinical data from different facilities conducting neutron capture therapy has continued since its founding at the 9th ISNCT symposium in October 2000. The thrust towards accumulating physical dosimetry data for comparison between different participants has broadened to include facilities in Japan and the determination of spectral descriptions of different beams. Retrospective analysis of patient data from the Brookhaven Medical Research Reactor is also being considered for incorporation into this study to increase the pool of available data. Meanwhile the next essential phase of comparing measurements of visiting dosimetry groups with treatment plan calculations from the host institutes has commenced. Host centers from Petten, Finland and the Czech Republic in Europe and MIT in the USA have applied the regular calculations and clinical calibrations from their current clinical studies, to generate treatment plans in the large standard phantom used for measurements by visiting participants. These data have been exchanged between the participants and scaling factors to relate the separate dose components between the different institutes are being determined. Preliminary normalization of measured and calculated dosimetry for patients is nearing completion to enable the physical radiation doses that comprise a treatment prescription at a host institute to be directly related to the corresponding measured doses of a visiting group. This should serve as an impetus for the direct comparison of patient data although the clinical requirements for achieving this need to be clearly defined. This may necessitate more extensive comparisons of treatment planning calculations through the solution of test problems and clarification regarding the question of dose specification from treatment calculations in general.

Tolerance of normal human brain to boron neutron capture therapy

Data from the Harvard-MIT and the BNL Phase I and Phase I/II clinical trials, conducted between 1994 and 1999, have been analyzed and combined, providing the most complete data set yet available on the tolerance of the normal human brain to BPA-mediated boron neutron capture therapy. Both peak (1cm(3)) dose and average whole-brain dose show a steep dose-response relationship using somnolence syndrome as the clinical endpoint. Probit analysis indicates that the doses associated with a 50% incidence for somnolence (ED(50)+/-SE) were 6.2+/-1.0 Gy(w) for average whole-brain dose and 14.1+/-1.8 Gy(w) for peak brain dose.

Effects of boron neutron capture irradiation on the normal lung of rats

The whole lung of rats was irradiated with X-rays, thermal neutrons, or thermal neutrons in the presence of p-boronophenylalanine (BPA). A >/= 20% increase in breathing rate, in the period 40-80 days after irradiation, was indicative of radiation-induced pneumonitis. The ED(50) (+/-SE) for a >/= 20% increase in breathing rate, relative to age-matched controls, was 11.6 +/- 0.13 Gy for X-rays and 9.6 +/- 0.08 Gy for neutrons only. This indicated a thermal neutron beam RBE of 1.2 and an RBE of 2.2 for the high-LET components of the dose, assuming a dose reduction factor of 1.0 for gamma rays. Preliminary data indicate the compound biological effectiveness factor for BPA in the lung is approximately 1.5.

A state-of-the-art epithermal neutron irradiation facility for neutron capture therapy

At the Massachusetts Institute of Technology (MIT) the first fission converter-based epithermal neutron beam (FCB) has proven suitable for use in clinical trials of boron neutron capture therapy (BNCT). The modern facility provides a high intensity beam together with low levels of contamination that is ideally suited for use with future, more selective boron delivery agents. Prescriptions for normal tissue tolerance doses consist of 2 or 3 fields lasting less than 10 min each with the currently available beam intensity, that are administered with an automated beam monitoring and control system to help ensure safety of the patient and staff alike. A quality assurance program ensures proper functioning of all instrumentation and safety interlocks as well as constancy of beam output relative to routine calibrations. Beam line shutters and the medical room walls provide sufficient shielding to enable access and use of the facility without affecting other experiments or normal operation of the multipurpose research reactor at MIT. Medical expertise and a large population in the greater Boston area are situated conveniently close to the university, which operates the research reactor 24 h a day for approximately 300 days per year. The operational characteristics of the facility closely match those established for conventional radiotherapy, which together with a near optimum beam performance ensure that the FCB is capable of determining whether the radiobiological promise of NCT can be realized in routine practice.

The design, construction and performance of a variable collimator for epithermal neutron capture therapy beams

A patient collimator for the fission converter based epithermal neutron beam (FCB) at the Massachusetts Institute of Technology Research Reactor (MITR-II) was built for clinical trials of boron neutron capture therapy (BNCT). A design was optimized by Monte Carlo simulations of the entire beam line and incorporates a modular construction for easy modifications in the future. The device was formed in-house by casting a mixture of lead spheres (7.6 mm diameter) in epoxy resin loaded with either 140 mg cm(-3) of boron carbide or 210 mg cm(-3) of lithium fluoride (95% enriched in 6Li). The cone shaped collimator allows easy field placement anywhere on the patient and is equipped with a laser indicator of central axis, beam's eye view optics and circular apertures of 80, 100, 120 and 160 mm diameter. Beam profiles and the collateral dose in a half-body phantom were measured for the 160 mm field using fission counters, activation foils as well as tissue equivalent (A-150) and graphite walled ionization chambers. Leakage radiation through the collimator contributes less than 10% to the total collateral dose up to 0.15 m beyond the edge of the aperture and becomes relatively more prominent with lateral displacement. The measured whole body dose equivalent of 24 +/- 2 mSv per Gy of therapeutic dose is comparable to doses received during conventional therapy and is due principally (60-80%) to thermal neutron capture reactions with boron. These findings, together with the dose distributions for the primary beam, demonstrate the suitability of this patient collimator for BNCT.

Dosimetric measurements with a brain equivalent plastic walled ionization chamber in an epithermal neutron beam

The tissue substitute A-181 plastic, which has an elemental composition matching both the constituent hydrogen and nitrogen of brain tissue, was assessed for dosimetry in boron neutron capture therapy (BNCT). The sensitivity of an A-181 walled ionization chamber relative to photons for all neutrons in a clinical epithermal beam was calculated to vary between 0.79 +/- 0.04 in-air and 0.95 +/- 0.01 at depths of 4 cm and greater in-phantom. Differences in the total neutron doses measured with A-150 and A-181 plastic-walled chambers were attributed, within experimental error, to the dose produced by thermal neutron capture reactions from the different concentrations of nitrogen in the two tissue substitutes. The response of the A-181 chamber was converted to total neutron dose with an uncertainty increasing with depth in-phantom from 13 to 23% the magnitude of which is determined by the subtraction of a relatively large photon dose. The use of A-181 in place of A-150 plastic will no longer require partitioning the measured neutron dose by energy and should simplify dose reporting in BNCT.

Performance characteristics of the MIT fission converter based epithermal neutron beam

A pre-clinical characterization of the first fission converter based epithermal neutron beam (FCB) designed for boron neutron capture therapy (BNCT) has been performed. Calculated design parameters describing the physical performance of the aluminium and Teflon filtered beam were confirmed from neutron fluence and absorbed dose rate measurements performed with activation foils and paired ionization chambers. The facility currently provides an epithermal neutron flux of 4.6 x 10(9) n cm(-2) s(-1) in-air at the patient position that makes it the most intense BNCT source in the world. This epithermal neutron flux is accompanied by very low specific photon and fast neutron absorbed doses of 3.5 +/- 0.5 and 1.4 +/- 0.2 x 10(-13) Gy cm2, respectively. A therapeutic dose rate of 1.7 RBE Gy min(-1) is achievable at the advantage depth of 97 mm when boronated phenylalanine (BPA) is used as the delivery agent, giving an average therapeutic ratio of 5.7. In clinical trials of normal tissue tolerance when using the FCB, the effective prescribed dose is due principally to neutron interactions with the nonselectively absorbed BPA present in brain. If an advanced compound is considered, the dose to brain would instead be predominately from the photon kerma induced by thermal neutron capture in hydrogen and advantage parameters of 0.88 Gy min(-1), 121 mm and 10.8 would be realized for the therapeutic dose rate, advantage depth and therapeutic ratio, respectively. This study confirms the success of a new approach to producing a high intensity, high purity epithermal neutron source that attains near optimal physical performance and which is well suited to exploit the next generation of boron delivery agents.

Pharamacokinetic modeling for boronophenylalanine-fructose mediated neutron capture therapy: 10B concentration predictions and dosimetric consequences

A two-compartment open model has been developed for predicting 10B concentrations in blood following intravenous infusion of the L-p-boronophenylalanine-fructose complex in humans and derived from pharmacokinetic studies of 24 patients in Phase I clinical trials of boron neutron capture therapy. The 10B concentration profile in blood exhibits a characteristic rise during the infusion to a peak of approximately 32 microg/g (for infusion of 350 mg/kg over 90 min) followed by a biexponential disposition profile with harmonic mean half-lives of 0.32 +/- 0.08 and 8.2 +/- 2.7 h, most likely due to redistribution and primarily renal elimination, respectively. The mean model rate constants k12, k21, and k10 are (mean +/- SD) 0.0227 +/- 0.0064 min(-1), 0.0099 +/- 0.0027 min(-1), 0.0052 +/- 0.0016 min(-1), respectively, and the central compartment volume of distribution V1 is 0.235 +/- 0.042 L/kg. In anticipation of the initiation of clinical trials using an intense neutron beam with concomitantly short irradiations, the ability of this model to predict, in advance, the average blood 10B concentration during brief irradiations was simulated in a retrospective analysis of the pharmacokinetic data from these patients. The prediction error for blood boron concentration and its effect on simulated dose delivered for each irradiation field are reported for three different prediction strategies. In this simulation, error in delivered dose (or, equivalently, neutron fluence) for a given single irradiation field resulting from error in predicted blood 10B concentration was limited to less than 10%. In practice, lower dose errors can be achieved by delivering each field in two fractions (on two separate days) and by adjusting the second fraction's dose to offset error in the first.

The validity of adult arrestee self-reports of crack cocaine use

Despite the many problems associated with crack use, little validated empirical evidence about the prevalence of crack cocaine exists. Researchers that track crack cocaine use have relied on self-reports to differentiate crack and powder cocaine. Prior research suggests that the accuracy of self-reports for the use of a variety of illicit substances is relatively low. To examine the validity of self-reports of crack use, this article employs a newly developed technology to detect specifically the presence of markers of crack cocaine in urine specimens. With a sample of 2327 arrestees from six cities that participate in the Arrestee Drug Abuse Monitoring (ADAM) Program, both face-to-face interview and urinalysis data were examined. Using a positive urinalysis result as the validity standard, we assessed the extent to which arrestees underreport crack cocaine use as compared to the use of marijuana, opiates,and methamphetamine. Logistic regression models were also de veloped to predict the factors that relate to underreporting. The results showed a considerable amount of underreporting for all the drug measures. In most cases, only about half the people who had a positive urinalysis test for drugs admitted using drugs. Overall, the least amount of underreporting occurred for the use of marijuana (63.6% told the "truth"), followed by methamphetamine (56.1% told the truth), crack (48.2% told the truth), and opiate (45.9% told the truth). Female crack users, as compared to male crack users, were more likely to admit using crack. Black arrestees were more likely to admit using crack than white or Hispanic arrestees. Arrestees with a history of prior drug treatment or a prior arrest, as compared to those without such histories, were more likely to admit using crack. The older the arrestee was, the more likely the arrestee would admit using crack. The more money an arrestee spent on drugs, the more likely the arrestee would admit using crack. Differences in underreporting were also observed across the six cities in this study. The implications of these findings for the monitoring of crack use are discussed.

A pharmacokinetic model for the concentration of 10B in blood after boronophenylalanine-fructose administration in humans

An open two-compartment model has been developed for predicting (10)B concentrations in blood after intravenous infusion of the l-p-boronophenylalanine-fructose complex (BPA-F) in humans and derived from studies of pharmacokinetics in 24 patients in the Harvard-MIT Phase I clinical trials of BNCT. The (10)B concentration profile in blood exhibits a characteristic rise during the infusion to a peak of approximately 32 microg/g (for infusion of 350 mg/kg over 90 min) followed by a biphasic exponential clearance profile with half-lives of 0.34 +/- 0.12 and 9.0 +/- 2.7 h, due to redistribution and primarily renal elimination, respectively. The model rate constants k(1), k(2) and k(3) are 0.0227 +/- 0.0064, 0.0099 +/- 0.0027 and 0.0052 +/- 0.0016 min(-1), respectively, and the central compartment volume of distribution, V(1), is 0.235 +/- 0.042 kg/kg. The validity of this model was demonstrated by successfully predicting the average pharmacokinetic response for a cohort of patients who were administered BPA-F using an infusion schedule different from those used to derive the parameters of the model. Furthermore, the mean parameters of the model do not differ for cohorts of patients infused using different schedules.

High-average-power operation of a Q-switched diode-pumped holmium laser

We have investigated high-peak- and high-average-power operation of diode-pumped, thulium-sensitized, holmium 2.1-microm lasers. Free-running laser powers of 14 W at 29 Hz have been demonstrated with 2.6% electrical efficiency. Q-switched operation produced average powers in excess of 11 W in a burst of short pulses. Preliminary optical parametric oscillator frequency conversion of the holmium laser to 4 microm is also reported.