Standardization of flow cytometric detection of antigen expression

Since response to antigen-based immunotherapy relies upon the level of tumor antigen expression we developed an antigen quantification assay using ABC values. Antigen quantification as a clinical assay requires methods for quality control and for interlaboratory and inter-cytometer platform standardization. A single lot of Cytotrol™ Lyophilized Control Cells (Beckman Coulter) used for all studies. The variability in antigen quantification across 4 different instrument platforms in 2 separate laboratories was evaluated. The effect of the antibody clone utilized, importance of custom 1:1 molar ratio (fluorophore to protein, F/P) verses off-the-shelf antibodies, and QuantiBrite PE calibration verses linearity calibration combined with a single point scale transformation with CD4 as reference were determined. Use of single lot control cells allowed validation of reproducibility between flow cytometer platforms and laboratories and allowed assessment of different antibody lots, cocktail preparation, and different antibody clones. Off the shelf antibody preparations provide reproducible estimates of antigen density, however custom 1:1 unimolar antibody preparations should be utilized for definitive measurement of antigen expression.Geometric Mean fluorescent Intensity (GeoMFI) was not comparable across instruments and inter-laboratory. The use of CD4 as the reference marker can minimize variability in ABC values. Comparable antigen quantification is vital in managing patients receiving antigen-based immunotherapy. If this assay is to be utilized in a clinical setting, quality control methods have to be instituted to assure reproducibility and allow validation across laboratories. We have demonstrated that use of a lyophilized cell control is highly valuable in achieveing these goals.

Expanding NIST Calibration of Fluorescent Microspheres for Flow Cytometry to More Fluorescence Channels and Smaller Particles

The National Institute of Standards and Technology (NIST), the National Institutes of Health (NIH) and other industry stakeholders have been working together to enable fluorescence intensities of flow cytometer calibration beads to be assigned quantitative equivalent reference fluorophore (ERF) values with high accuracy and precision. The ultimate goal of this effort is to accurately quantify the number of antibodies bound to individual living cells. The expansion of this effort to assign ERF values to more than 50 fluorescence channels and particles with diameters ranging from 10 μm down to 80 nm is reported here.

The NISTmAb Reference Material 8671 value assignment, homogeneity, and stability

The NISTmAb Reference Material (RM) 8671 is intended to be an industry standard monoclonal antibody for pre-competitive harmonization of best practices and designing next generation characterization technologies for identity, quality, and stability testing. It must therefore embody the quality and characteristics of a typical biopharmaceutical product and be available long-term in a stable format with consistent product quality attributes. A stratified sampling and analysis plan using a series of qualified analytical and biophysical methods is described that assures RM 8671 meets these criteria. Results for the first three lots of RM 8671 highlight the consistency of material attributes with respect to size, charge, and identity. RM 8671 was verified to be homogeneous both within and between vialing lots, demonstrating the robustness of the lifecycle management plan. It was analyzed in concert with the in-house primary sample 8670 (PS 8670) to provide a historical link to this seminal material. RM 8671 was verified to be fit for its intended purpose as a technology innovation tool, external system suitability control, and cross-industry harmonization platform. Graphical abstract The NISTmAb Reference Material (RM) 8671 is intended to be an industry standard monoclonal antibody for pre-competitive harmonization of best practices and designing next generation characterization technologies for identity, quality, and stability testing.

Limitations of methods for measuring the concentration of human genomic DNA and oligonucleotide samples

We compared different methods (absorbance, fluorescent dye-binding, and digital PCR) for measuring the concentrations of human genomic DNA from cultured cells and absorbance measurements of a synthetic DNA oligonucleotide. NIST Standard Reference Material (SRM) 2082, a pathlength absorbance standard, was used to benchmark the absorbance measurements done with microvolume spectrophotometers and a microvolume plate reader. Control absorbance values were measured on a high accuracy spectrophotometer and a NIST calibrated pathlength cuvette. Measurements of the human genomic DNA sample were done with several types of fluorescent dye binding assays using different DNA calibrators. The fluorescent dye binding methods gave different results for genomic DNA depending upon the type of DNA calibrator and the fluorescent dye that was used. The human genomic DNA sample was also characterized by using six different droplet digital PCR assays (amplicons located on different chromosomes) to measure the average copy number. Conversion of the digital PCR data to copy numbers was sensitive to the droplet size used for calculations and conversion to mass concentration was dependent upon the molecular weight of the human genome used for the calculations. The results from the different methods were compared and the caveats for each measurement method were discussed.

NIST Spectroscopic Measurement Standards

Ultraviolet (UV) absorbance measurements provide a rapid and reliable method to determine protein concentrations. the National Institute of standards and technology (NIST) has developed Standard Reference Material (SRM) 2082 as a pathlength standard for UV absorbance measurements for use with the new generation of microvolume spectrophotometers and short-pathlength cuvettes. short pathlengths are used with high-concentration targets to ensure that absorbance values are within the optimal range. the short-pathlength instruments and cuvettes also reduce the required volumes to conserve valuable samples. the authors compared the results obtained with high-quality dual-beam spectrophotometers and short-pathlength cuvettes to the results obtained from a microvolume spectrophotometer and a microvolume plate reader. SRM 2082 can be used to accurately calculate pathlength values, thereby increasing the accuracy in subsequent measurements using the short-pathlength cuvettes and microvolume absorbance instruments. RM 8671 (reference material, the NISTmAb) can then be used to ensure the accuracy and reproducibility of protein concentration measurements by providing an industrially relevant reference material, a well-characterized monoclonal antibody.

Development of NIST Standard Reference Material 2082, a Pathlength Standard for Measurements in the Ultraviolet Spectrum

New spectrophotometers and cuvettes have been designed to allow the measurement of absorbance values from samples using microliter volume sizes. These measurements are done using short pathlengths to decrease the sample volumes required. The major applications for these spectrophotometers and cuvettes are samples that are difficult to obtain in large amounts, such as proteins and nucleic acids that absorb light in the ultraviolet range. Existing ultraviolet absorbance standards have been designed for longer pathlength measurements. Standard Reference Material (SRM) 2082 was developed to validate the pathlengths of short-pathlength cuvettes and instruments using materials with absorbance spectra that are similar to the most commonly used samples. SRM 2082 consists of three individual components: a blank buffer solution, a solution of the amino acid tryptophan in the buffer, and a solution of the nucleobase uracil in the buffer. The tryptophan solution has an absorbance spectrum (peak at 280 nm) similar to proteins, and the uracil has an absorbance spectrum (peak at 260 nm) similar to nucleic acids. The absorbance values of these solutions were determined using a series of cuvettes with pathlengths from 0.1 mm to 2 mm. The pathlengths of the cuvettes used for the absorbance measurements were determined at the National Institute of Standards and Technology by physical and optical measurements. The effects of temperature and spectral bandwidth variations on the absorbance values of SRM 2082 were also investigated.

Assignment of the Number of Equivalent Reference Fluorophores to Dyed Microspheres

A procedure will be described to assign to each dyed microsphere a number called the Equivalent number of Reference Fluorophores (ERF). The ERF unit gives the number of reference fluorophores in solution which produce the same fluorescence signal as a single dyed microsphere. In the first step, fluorescence measurements were carried out on serial dilutions of a solution of reference fluorophores. The resulting fluorescence intensities and the corresponding concentrations were used to calibrate the response of the fluorometer. The calibration consisted of establishing a linear relation between the intensities and concentrations. In the second step, the fluorescence intensity from a suspension of microspheres was measured in order to determine the equivalent concentration of reference fluorophores which gave the same fluorescence intensity as the suspension of microspheres. This was performed by utilizing the calibration line obtained in the first step. In the third step, a flow cytometer and a light obscuration apparatus were used to measure the total concentration of microspheres in the suspensions used for the fluorescence measurements. In addition to the total microsphere concentration, the flow cytometer also enabled the measurement of the concentration of a sub population of microspheres which are used to calibrate the fluorescence scale of a flow cytometer. The fourth step utilized the data collected in steps one, two, and three to assign a value of ERF to individual microspheres. The set of microspheres with assigned ERF values will be used to establish a linear fluorescence scale in each channel of a flow cytometer. The discussion will emphasize the estimate of uncertainties in each step of the assignment process.

Optical Properties of CdSe/ZnS Nanocrystals

Measurements are presented of the absorbance, fluorescence emission, fluorescence quantum yield, and fluorescence lifetime of CdSe/ZnS nanocrystals, also known as quantum dots (QDs). The study included three groups of nanocrystals whose surfaces were either passivated with organic molecules, modified further with carboxyl groups, or conjugated with CD14 mouse anti-human antibodies. The surface modifications had observable effects on the optical properties of the nanocrystals. The oscillator strength (OS) of the band edge transition was about 1.0 for the nanocrystals emitting at 565 nm, 605 nm, and 655 nm. The OS could not be determined for QDs with emission at 700 nm and 800 nm. The fluorescence lifetimes varied from 26 ns for nanocrystals emitting near 600 nm to 150 ns for nanocrystals emitting near 800 nm. The quantum yield ranged between 0.4 and 0.9 for the nanocrystals in this study. A brightness index (BI) was used to evaluate the suitability of the nanocrystal labels for flow cytometer measurements. Most QD labels are at least as bright as fluorescein for applications in flow cytometer assays with 488 nm excitation. For optimal brightness the QDs should be excited with 405 nm light. We observed a strong dependence of the QD absorbance at 250 nm on the surface modification of the QD.

Development of bavituximab, a vascular targeting agent with immune-modulating properties, for lung cancer treatment

Bavituximab is a chimeric monoclonal antibody directed against the membrane phospholipid phosphatidylserine. Phosphatidylserine exposure is increased on endothelial cells and apoptotic cancer cells in solid tumors, allowing tumor-specific targeting of bavituximab. Bavituximab binding results in tumor vessel occlusion and enhanced antitumor immunity. Preclinical investigations have demonstrated efficacy as monotherapy and in combination with other modalities against multiple cancer types. Phase I clinical trials of bavituximab monotherapy and in combination with chemotherapy in adults with refractory solid tumors have been completed. Phase II trials of bavituximab in combination with chemotherapy for the first- and second-line treatment of advanced non-small-cell lung cancer are currently ongoing. This article summarizes the preclinical development and clinical experience with bavituximab in non-small-cell lung cancer.

Phase I dose-escalation study of stereotactic body radiation therapy for low- and intermediate-risk prostate cancer

PURPOSE

To evaluate the tolerability of escalating doses of stereotactic body radiation therapy in the treatment of localized prostate cancer.

PATIENTS AND METHODS

Eligible patients included those with Gleason score 2 to 6 with prostate-specific antigen (PSA) ≤ 20, Gleason score 7 with PSA ≤ 15, ≤ T2b, prostate size ≤ 60 cm(3), and American Urological Association (AUA) score ≤ 15. Pretreatment preparation required an enema and placement of a rectal balloon. Dose-limiting toxicity (DLT) was defined as grade 3 or worse GI/genitourinary (GU) toxicity by Common Terminology Criteria of Adverse Events (version 3). Patients completed quality-of-life questionnaires at defined intervals.

RESULTS

Groups of 15 patients received 45 Gy, 47.5 Gy, and 50 Gy in five fractions (45 total patients). The median follow-up is 30 months (range, 3 to 36 months), 18 months (range, 0 to 30 months), and 12 months (range, 3 to 18 months) for the 45 Gy, 47.5 Gy, and 50 Gy groups, respectively. For all patients, GI grade ≥ 2 and grade ≥ 3 toxicity occurred in 18% and 2%, respectively, and GU grade ≥ 2 and grade ≥ 3 toxicity occurred in 31% and 4%, respectively. Mean AUA scores increased significantly from baseline in the 47.5-Gy dose level (P = .002) as compared with the other dose levels, where mean values returned to baseline. Rectal quality-of-life scores (Expanded Prostate Cancer Index Composite) fell from baseline up to 12 months but trended back at 18 months. In all patients, PSA control is 100% by the nadir + 2 ng/mL failure definition.

CONCLUSION

Dose escalation to 50 Gy has been completed without DLT. A multicenter phase II trial is underway treating patients to 50 Gy in five fractions to further evaluate this experimental therapy.

Procedures for Wavelength Calibration and Spectral Response Correction of CCD Array Spectrometers

This work describes a procedure for acquiring a spectrum of an analyte over an extended range of wavelengths and validating the wavelength and intensity assignments. To acquire a spectrum over an extended range of wavelengths with a spectrometer with a charge coupled device (CCD) array detector, it is necessary to acquire many partial spectra, each at a different angular position of the grating, and splice the partial spectra into a single extended spectrum. The splicing procedure exposes instrument dependent artifacts. It is demonstrated that by taking a spectrum of a reference irradiance source and making spectral correction, the artifacts exposed by the splicing are removed from the analyte spectrum. This is because the irradiance reference spectrum contains the same artifacts as the analyte spectrum. The artifacts exposed by the splicing depend on the wavelength of the splice; therefore it is important to measure the irradiance reference spectrum for the same range of wavelengths used to measure the spectrum of the analyte solution. In other words, there is no general spectral correction factor which is applicable to spectra taken for different range of wavelengths. The wavelength calibration is also carried out by splicing many partial spectra from a source like a krypton lamp. However the wavelength assignments are not sensitive to the splicing procedure and the same wavelength calibration can be used for spectra acquired over different extended wavelength ranges. The wavelength calibration checks the validity of the setting of the grating angular position, and the assignment of wavelengths to individual pixels on the CCD array detector. The procedure is illustrated by measuring the spectrum of an orange glass and the spectrum of a suspension of microalgae.

Photophysical Properties of Ricin

The molar absorption coefficient of ricin in phosphate-buffered saline (PBS) at 279 nm was measured as (93,900+/-3300) L mol(-1) cm(-1). The concentration of ricin was determined using amino acid analysis. The absorption spectrum of ricin was interpreted in terms of 69% contribution from absorption by tryptophan residues and 31% contribution from absorption by tyrosine residues. The total dipole strength of the ricin band at 280 nm was determined to be (147+/-8) D2 and was consistent with the combined dipole strengths of 10 tryptophan ([11.7+/-1.0] D2) and 23 tyrosine ([1.4+/-0.2] D2) residues. The structure of ricin was used to determine the coupling of the tryptophan residues in ricin. The maximum interaction energy was found to be 424 cm(-1)/epsilon while the average interaction between any two pairs of tryptophan residues was approximately 18 cm(-1)/epsilon. In this study, epsilon is the dielectric constant inside the protein. The fluorescence from ricin, excited at 280 nm, was dominated by fluorescence from tryptophan residues suggesting the presence of energy transfer from tyrosine to tryptophan residues. The absorbance and fluorescence of ricin increased slightly when ricin was denatured in a high concentration of guanidine. Irreversible thermal unfolding of ricin occurred between 65 degrees C and 70 degrees C. (D=3.3364*10(-30) Cm, not SI unit, convenient unit for the magnitude of the electric dipole moment of molecules.).

Cathepsin-D and tumor associated antigen DF3 in salivary gland neoplasia. Differential diagnostic and prognostic applications

Salivary gland tumors pose considerable difficulty in diagnostic and prognostic assessment based on the histopathologic features alone. Cathepsin-D is overexpressed in cancer cells where its concentration in the primary tumor is correlated with increased risk of metastasis. DF3 antigen is a tumor associated glycoprotein that is specific for malignant cells of glandular origin. We examined the distribution patterns of cathepsin-D and DF3 antigens in benign (n = 11) and malignant (n = 44) salivary gland tumors of various histologic types. The frequency of cathepsin-D expression is significantly increased (p < 0.001) in salivary gland carcinomas compared to benign mixed tumors (BMT). High levels of cathepsin-D expression was frequent in carcinomas ex BMT, mucoepidermoid carcinomas, poorly differentiated adenocarcinomas-NOS and adenoid cystic carcinomas (ACC). Acinic cell carcinomas and polymorphous low-grade adenocarcinomas were mostly negative. Intense cytoplasmic staining for DF3 antigen was noted in the tumor cells of mucoepidermoid carcinomas, carcinomas ex BMT and poorly differentiated adenocarcinomas-NOS whereas other types of salivary gland carcinomas exhibited either negative or only focal membrane staining. The noted differences in the reactive patterns of cathepsin-D and DF3 antigen among various histologic types of salivary gland carcinomas may have differential diagnostic and prognostic applications.