We are thankful to University of Virginia Cancer Center Core Imaging Facility, Biomolecular Analysis Facility, Advanced Microscopy Facility and the Core Vivarium Facility for Assistance. J. T-S is an early career investigator of Ovarian Cancer Academy (OCA-DoD). This work was supported by NCI/NIH grant (R01CA233752) to J. T-S, U.S. DoD Breast Cancer Research Program (BCRP) breakthrough level-1 award to J. T-S (BC17097) and U.S. DoD Ovarian Cancer Research Program (OCRP) funding award (OC180412) to J. T-S

All the procedures involving animals handling and tumor xenografts studies were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) here at the University of Virginia and conform to the relevant regulatory standards
1. Expression and purification of antibodies
<ol>
	<li>Maintenance of CHO cells
	<ol>
		<li>Grow CHO cells in FreeStyle CHO Media supplemented with commercially available 1x glutamine supplement at 37 &#730;C shaking at 130 rpm with 5% CO2 ...

<ol>
	<li>Takahashi, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Muromonab+CD3+(Orthoclone+OKT3).">Muromonab CD3 (Orthoclone OKT3).</a> Journal of Toxicological Sciences. 20, 483-484 (1995).</li><li>Tushir-Singh, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Antibody-siRNA+conjugates:+drugging+the+undruggable+for+anti-leukemic+therapy.">Antibody-siRNA conjugates: drugging the undruggable for anti-leukemic therapy.</a> Expert Opinion in Biological Therapy. 17, 325-338 (2017).</li><li>Gravitz, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cancer+immunotherapy.">Cancer immunotherapy.</a> Nature. 504, 1 (2013).</li><li>Pagel, J. M., West, H. 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R., John, V., Seetharamu, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bi-specific+and+tri-specific+antibodies-+the+next+big+thing+in+solid+tumor+therapeutics.">Bi-specific and tri-specific antibodies- the next big thing in solid tumor therapeutics.</a> Molecular Medicine. 24, 50 (2018).</li><li>Shivange, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Single-Agent+Dual-Specificity+Targeting+of+FOLR1+and+DR5+as+an+Effective+Strategy+for+Ovarian+Cancer.">A Single-Agent Dual-Specificity Targeting of FOLR1 and DR5 as an Effective Strategy for Ovarian Cancer.</a> Cancer Cell. 34, 331-345 (2018).</li><li>Wajant, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+Mode+of+Action+of+TRAIL+Receptor+Agonists-Common+Principles+and+Their+Translational+Exploitation.">Molecular Mode of Action of TRAIL Receptor Agonists-Common Principles and Their Translational Exploitation.</a> Cancers (Basel). 11 (7), 954 (2019).</li><li>Necela, B. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Folate+receptor-alpha+(FOLR1)+expression+and+function+in+triple+negative+tumors.">Folate receptor-alpha (FOLR1) expression and function in triple negative tumors.</a> PLoS One. 10, 0122209 (2015).</li><li>Lin, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+antitumor+activity+of+the+human+FOLR1-specific+monoclonal+antibody,+farletuzumab,+in+an+ovarian+cancer+mouse+model+is+mediated+by+antibody-dependent+cellular+cytotoxicity.">The antitumor activity of the human FOLR1-specific monoclonal antibody, farletuzumab, in an ovarian cancer mouse model is mediated by antibody-dependent cellular cytotoxicity.</a> Cancer Biology Therapy. 14, 1032-1038 (2013).</li><li>Chen, Y., Kim, M. T., Zheng, L., Deperalta, G., Jacobson, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+Characterization+of+Cross-Linked+Species+in+Trastuzumab+Emtansine+(Kadcyla).">Structural Characterization of Cross-Linked Species in Trastuzumab Emtansine (Kadcyla).</a> Bioconjugate Chemistry. 27, 2037-2047 (2016).</li><li>Bauerschlag, D. O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anti-idiotypic+antibody+abagovomab+in+advanced+ovarian+cancer.">Anti-idiotypic antibody abagovomab in advanced ovarian cancer.</a> Future Oncology. 4, 769-773 (2008).</li><li>Narita, Y., Muro, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Challenges+in+molecular+targeted+therapy+for+gastric+cancer:+considerations+for+efficacy+and+safety.">Challenges in molecular targeted therapy for gastric cancer: considerations for efficacy and safety.</a> Expert Opinion in Drug Safety. 16, 319-327 (2017).</li><li>Jordan, N. V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=HER2+expression+identifies+dynamic+functional+states+within+circulating+breast+cancer+cells.">HER2 expression identifies dynamic functional states within circulating breast cancer cells.</a> Nature. 537, 102-106 (2016).</li><li>Fakih, M., Vincent, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adverse+events+associated+with+anti-EGFR+therapies+for+the+treatment+of+metastatic+colorectal+cancer.">Adverse events associated with anti-EGFR therapies for the treatment of metastatic colorectal cancer.</a> Current Oncology. 17, 18-30 (2010).</li><li>Koba, W., Jelicks, L. A., Fine, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=MicroPET/SPECT/CT+imaging+of+small+animal+models+of+disease.">MicroPET/SPECT/CT imaging of small animal models of disease.</a> American Journal of Pathology. 182, 319-324 (2013).</li><li>van der Wall, E. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cost+analysis+favours+SPECT+over+PET+and+CTA+for+evaluation+of+coronary+artery+disease:+the+SPARC+study.">Cost analysis favours SPECT over PET and CTA for evaluation of coronary artery disease: the SPARC study.</a> Netherland Heart Journal. 22, 257-258 (2014).</li><li>Van Dort, M. E., Rehemtulla, A., Ross, B. 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Selective and tumor tissue specific delivery of anti-cancer therapeutic agent is the key to measure efficacy and safety of a given targeted therapy13. Here we have described a quick and efficient approach to investigate the detailed tissue and tumor distribution of clinical, farletuzumab and a nonclinical BaCa antibody. The described approach is applicable to any newly generated antibody and can be used alongside of a clinically effective antibody (with desired qualities) for its tumor/organ distr...

FDA approved the first monoclonal antibody targeting CD3 (OKT3, Muromonab) in 19861,2. Since then for the next twenty years, there has been a rapid explosion in the field of antibody engineering due to the overwhelming success of antibodies against immune checkpoint inhibitors3. Beside indirect activation of immune system, antibodies are being aimed to directly flag cancer cells to precisely engage immune effector cells, trigger cytotoxicity via death receptor agonist, block tumor cell survival signaling, obstruct angiogenesis (growth of blood vessels), constrain immune checkpoint regulators, deliver radioisotopes, chemotherapy drugs and siRNA as a conjugated agents2. In addition, studying the single chain variable fragments (scFv) of various antibodies on the surface of patient derived T-cells and NK cells (CAR-T and CAR-NK) is a fast growing area of clinical investigations for cell-based therapies4.
The ultra-high affinity of antibody-based drugs that provides selectivity to antigen expressing tumor cells makes it an attractive agent. Likewise, the targeted delivery and tumor retention of a therapeutic antibody (or a chemical drug) is the key to balance efficacy over toxicity. Therefore, a large number of protein engineering based strategies that include but are not limited to bispecific5 and tri-specific antibodies6 are being exploited to significantly enhance avidity optimized tumor retention of intravenously (IV) injected therapeutics5,7. Here, we describe a simple fluorescence-based method to address the tumor and tissue distribution of potentially effective anti-cancer antibodies.
Because animal tissues possess auto-fluorescence when excited in visible spectrum, the antibodies were initially labeled with near Infrared dye (e.g., IRDye 800CW). For proofs of concept investigations, we have made use of folate receptor alpha-1 (FOLR1) targeting antibody called farletuzumab and its derivative called Bispecific anchor Cytotoxicity activator (BaCa)7 antibody that co-targets FOLR1 and death receptor-5 (DR5)8 in one recombinant antibody. FOLR1 is a well-defined overexpressed target receptor in ovarian and TNBC cancer cells, tumor xenografts and patient tumors9. Notably, there are multiple efforts to clinically exploit FOLR1 using antibody-based approaches to engage immune effector cells and antibody drug conjugates (ADC) for ovarian and breast cancers10,11.
In this methods paper, we cloned, expressed and purified clinical anti-FOLR1 (farletuzumab) along with other control antibodies using CHO expression system. IgG1 isotype and a clinical anti-idiotype mucin-16 antibody called abagovomab12 were used as negative controls. Following protein-A purification, indicated antibodies were labeled with IRDye 800CW and were administered into the tail vein of nude mice either bearing ovarian tumor xenografts or stably transfected human FOLR1 expressing murine colon cancer xenografts. The antibody localization was tracked by live imaging using in vivo imaging spectrum at multiple different time points7. This method does not require any genetic modification or injection of the substrate to enable light emission and is significantly quicker, cost effective and efficient. The general cloning, expression, purification and labeling protocol described below can be applied to any clinical and nonclinical antibody if heavy and light chain sequences are available.

<table>
	<tbody>
		<tr>
			<td>FreeStyle CHO media</td>
			<td>Gibco Life Technologies</td>
			<td>Cat # 12651-014</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-Anti (100X)</td>
			<td>Gibco Life Technologies</td>
			<td>Cat # 15240-062</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-Clumping Agent</td>
			<td>Gibco Life Technologies</td>
			<td>Cat # 01-0057DG</td>
			<td></td>
		</tr>
		<tr>
			<td>BD Insulin Syringe</td>
			<td>BD BioSciences</td>
			<td>Cat #329420</td>
			<td></td>
		</tr>
		<tr>
			<td>Caliper IVIS Spectrum</td>
			<td>PerkinElmer</td>
			<td>Cat #124262</td>
			<td></td>
		</tr>
		<tr>
			<td>CHO CD EfficientFeed B</td>
			<td>Gibco Life Technologies</td>
			<td>Cat #A10240-01</td>
			<td></td>
		</tr>
		<tr>
			<td>Corning 500 mL DMEM (Dulbecco&#39;s Modified Eagle&#39;s Medium)</td>
			<td>Corning</td>
			<td>Cat # 10-13-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>Corning 500 mL RPMI 1640</td>
			<td>Corning</td>
			<td>Cat # 10-040-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>Cy5 conjugated Anti-Human IgG (H+L)</td>
			<td>Jackson ImmunoResearch</td>
			<td>Cat # 709-175-149</td>
			<td></td>
		</tr>
		<tr>
			<td>GlutaMax-I (100X)</td>
			<td>Gibco Life Technologies</td>
			<td>Cat # 35050-061</td>
			<td></td>
		</tr>
		<tr>
			<td>HiPure Plasmid Maxiprep kit</td>
			<td>Invitrogen</td>
			<td>Cat # K21007</td>
			<td></td>
		</tr>
		<tr>
			<td>HiTrap MabSelect SuRe Column</td>
			<td>GE Healthcare</td>
			<td>Cat # 11-0034-93</td>
			<td></td>
		</tr>
		<tr>
			<td>Infusion</td>
			<td>Takara BioScience</td>
			<td>STO344</td>
			<td></td>
		</tr>
		<tr>
			<td>IRDye 800CW NHS Ester</td>
			<td>LI-COR</td>
			<td>Cat # 929-70020</td>
			<td></td>
		</tr>
		<tr>
			<td>Isoflurane, USP</td>
			<td>Covetrus</td>
			<td>Cat # 11695-6777-2</td>
			<td></td>
		</tr>
		<tr>
			<td>Lubricant Eye Ointment</td>
			<td>Refresh Lacri-Lube</td>
			<td>Cat #4089</td>
			<td></td>
		</tr>
		<tr>
			<td>Matrigel</td>
			<td>Corning</td>
			<td>Cat # 354234</td>
			<td></td>
		</tr>
		<tr>
			<td>PEI transfection reagent</td>
			<td>Thermo Fisher</td>
			<td>Cat # BMS1003A</td>
			<td></td>
		</tr>
		<tr>
			<td>Slide-A-Lyzer Dialysis Cassettes</td>
			<td>Thermo Scientific</td>
			<td>Cat # 66333</td>
			<td></td>
		</tr>
		<tr>
			<td>Steritop Vacuum Filters</td>
			<td>Millipore Express</td>
			<td>Cat #S2GPT02RE</td>
			<td></td>
		</tr>
		<tr>
			<td>Trypsin-EDTA</td>
			<td>Gibco Life Technologies</td>
			<td>Cat # 15400-054</td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Experimental Models: Cell lines</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Human: OVCAR-3</td>
			<td>American Type Culture Collection</td>
			<td>ATCC HTB-161</td>
			<td></td>
		</tr>
		<tr>
			<td>Human: CHO-K cells</td>
			<td>Stable transformed in our lab</td>
			<td>ATCC CCL-61</td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse: 4T1</td>
			<td>Kind gift from Dr. Chip Landen, UVA</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse: MC38</td>
			<td>Kind gift from Dr. Suzanne Ostrand-Rosenberg, UMBC</td>
			<td>Authenticated by STR profiling</td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse: MC38 hFOLR1</td>
			<td>Generated in our laboratory (This paper)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Experimental Models: Animal</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Mice: athymic Nude Foxn1nu/Foxn1+</td>
			<td>Envigo</td>
			<td>Multiple Orders</td>
			<td></td>
		</tr>
		<tr>
			<td>Mice: NOD.Cg Prkdcscid Il2rgtm1Wjl/SzJ</td>
			<td>Jackson Laboratory</td>
			<td>Multiple Orders</td>
			<td></td>
		</tr>
	</tbody>
</table>

analyzing tumor and tissue distribution of target antigen specific therapeutic antibody

Monoclonal antibodies are high affinity multifunctional drugs that work by variable independent mechanisms to eliminate cancer cells. Over the last few decades, the field of antibody-drug conjugates, bispecific antibodies, chimeric antigen receptors (CAR) and cancer immunotherapy has emerged as the most promising area of basic and therapeutic investigations. With numerous successful human trials targeting immune checkpoint receptors and CAR-T cells in leukemia and melanoma at a breakthrough pace, it is highly exciting times for oncologic therapeutics derived from variations of antibody engineering. Regrettably, a significantly large numbers of antibody and CAR based therapeutics have also proven disappointing in human trials of solid cancers because of the limited availability of immune effector cells in the tumor bed. Importantly, nonspecific distribution of therapeutic antibodies in tissues other than tumors also contribute to the lack of clinical efficacy, associated toxicity and clinical failure. As faithful translation of preclinical studies into human clinical trails are highly relied on mice tumor xenograft efficacy and safety studies, here we highlight a method to test the tumor and general tissue distribution of therapeutic antibodies. This is achieved by labeling the protein-A purified antibody with near Infrared fluorescent dye followed by live imaging of tumor bearing mice.

In the described methodology, first we cloned antibodies targeting folate receptor alpha-1 (FOLR1) named farletuzumab, and a bispecific antibody called BaCa consisting of farletuzumab and lexatumumab along with control antibodies such as abagovomab (sequences provided in Supplementary File 1). Details of representative variable heavy (VH) and variable light (VL) domains in DNA clones (pVH, pVL) are shown in Figure 1A. To confirm the positive clones, we carried out colony PCR...

Watch this Scientific Journal Video about Analyzing Tumor and Tissue Distribution of Target Antigen Specific Therapeutic Antibody at JoVE.com

Analyzing Tumor and Tissue Distribution of Target Antigen Specific Therapeutic Antibody

Here we present a protocol to study the in vivo localization of antibodies in mice tumor xenograft models.

Monoclonal antibodies are high affinity multifunctional drugs that work by variable independent mechanisms to eliminate cancer cells. Over the last few ...

This protocol demonstrates the expression and purification of antibodies and real time monitoring of antibody distribution in tumor and tissue. Screening antibodies with described protocol will help select antibodies that have limited non-specific tissue distribution, and enrich tumor localizations. The described technique is quick and cost-effective.Other techniques, such as single-photon emission computed tomography and positron emission tomography, are very expensive and require radioactive and other sophisticated tracers. The technique is not limited to cancer targeting antibodies. It can be applied to any other disease, such as lung or heart diseases, for the analysis of antibody distribution in the body.Demonstrating the procedure, will be Jeremy Gatesman, a veterinary technician from my university. Raw CHO cells in 200 milliliters of media, in Delong, Erlenmeyer baffled flasks. Combine five milliliters of CHO freestyle media with 50 micrograms of variable heavy clone DNA, and 75 micrograms a variable light clone DNA in a 15 milliliter tube.Then, vortex the tube. Leave the mixture at room temperature for five minutes. Add 750 microliters of one milligram per milliliter polyethylenimine stock to the DNA solution, and aggressively vortex it for 30 seconds.Leave the mixture at room temperature for an additional five minutes. Add the entire mixture to the CHO cells while manually shaking the flask. Immediately incubate the cells at 37 degrees Celsius while shaking at 130 RPM.On the next day, add two milliliters of 100 X anti-clumping agent, and two milliliters of 100 X antibacterial antimycotic solution to the cells. Incubate the flasks at 32 to 34 degrees Celsius, with shaking at 130 RPM. Continue culturing the cells for 10 to 11 days, adding 10 milliliters of Tryptone N1 feed, and two milliliters of 100 X glutamine supplement on every fifth day.Count the cells on every third day to ensure that cell viability remains above 80%On day 10 or 11, harvest the medium for antibody purification by centrifuging the culture at 3000 times G, and four degrees Celsius for 40 to 60 minutes. Then, filter the clear media with a 0.22 micrometer bottle filter. To purify the antibodies, equilibrate the protein A column with two column volumes of binding buffer.Pass the filtered media through the column at the flow rate of one milliliter per minute. Then wash the column with two column volumes of binding buffer. Use five milliliters of elution buffer to elute the antibody into 500 microliter fractions.And neutralize the pH of the alluded antibody by adding 10 microliters of neutralization buffer for fraction. Measure the concentration of the purified antibody with a spectrophotometer using the default protocol for IgG. Dialyze 0.5 milliliters of the antibody using a dialysis cassette in one liter of conjugation buffer.After four hours, transfer the dialysis cassette to fresh buffer and dialyze overnight. To set up a conjugation reaction, add 0.03 micrograms of IRR dye 800 CWU per one milligram of antibody in a total volume of 500 microliters. Incubate the mixture for two hours at 20 degrees Celsius, then purify the labeled conjugates by extensive dialysis against PBS.Estimate the degree of labeling by measuring the absorbance of the dye 780 nanometers, and absorbance of the protein at 280 nanometers. To develop mouse xenographs, gently lift the skin of the animal and separate it from the underlying muscle layer. Slowly inject 100 microliters of cell suspension under the skin with a 26 gauge needle.Wait for a few seconds before taking the needle out so that basement matrix medium inform the semi-solid gel-like structure along with cells under the skin, which will prevent the mixture from coming out of the injection site. To perform in vivo imaging, inject the dye labeled antibody via the tail vein. After anesthetizing the mouse, check for the lack of response to pedal reflexes, and dilate the vein by applying warm water.Use a 1cc insulin syringe with a 26 gauge needle to inject 25 micrograms of the labeled antibody in a volume of 100 microliters. As a negative control, label and inject the non-specific IgG isotype antibody which does not target cancer cells. Perform in vivo imaging eight, 24 and 48 hours after antibody injections.In the imaging software, click initialize. Confirm that the stage temperature is 37 degrees Celsius. In the control panel, set up fluorescence imaging through the imaging wizard, and set the excitation to 773 nanometers, emission to 792 nanometers.Transfer the anesthetized mouse into the imaging chamber. Position it on the imaging field using the nose cone. When ready, click acquire on the control panel for the image acquisition, and click auto expose.The generated image is the overlay of the fluorescence on the photographic image with optical fluorescence and density displayed in units of counts or photons. Positive cDNA clones with variable heavy and variable light domains were confirmed with colony PCR. Representative results of purified farletuzumab run on non-reducing and reducing SDS page are shown here.Heavy and light chains produced 50 and 25 kilodalton bands after reduction. Binding of antibodies to native proteins in the cell surface was also confirmed. Fluorescently labeled antibodies were tail vein injected, into mice grafted with full R1 expressing tumors.Animals were lived image at multiple time points using IVIS. the data confirs selective enrichment of full R1 and BaCa antibodies into the tumors. When attempting this protocol, keep in mind that held maintenance of CHO cells is critical for antibody yield.And it is important to have a similar degree of fluorescent dye conjugation for competitive tumor localization. Following this procedure, other methods such as tissue immunohistochemistry and tissue or tumor specific ELISA can be performed to support the live mice imaging data.

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Research

JoVE Journal

Cancer Research

5.1K visualizações.  University of Virginia School of Medicine. Este protocolo demonstra a expressão e purificação de anticorpos e o monitoramento em tempo real da distribuição de anticorpos em tumor e tecido. Os anticorpos de triagem com o protocolo descrito ajudarão a selecionar anticorpos que tenham distribuição limitada de tecidos não específicos e enriquecer as localizações tumorais. A técnica descrita é rápida e econômica.Outras técnicas, como tomografia computadorizada por emissão de fótons e tomografia de emissão de pós...