My clinical interests lie predominantly in treating rare tumors. I have special interest in diagnosing and treat unknown primary cancers (occult primary carcinoma), sarcoma and refractory ovarian cancers. I also am deeply involved in developmental therapeutics and early phase clinical studies. Currently I am running a phase I study of CDX-1127, a T-cel activating therapy in, B cell malignancies, renal cell carcinoma, and melanoma. Outside of my clinical work, I collaborate with Dr. Branimir Sikic on developing predictive biomarkers in ovarian cancers.

Clinical Focus

  • Neoplasms, Unknown Primary
  • Neoplasm, Occult Primary
  • Sarcoma
  • Ovarian Cancer
  • Medical Oncology
  • Clinical Trial, Phase I

Academic Appointments

Administrative Appointments

  • Instructor of Internal Medicine, Division of Medical Oncology, Stanford University (2013 - Present)

Boards, Advisory Committees, Professional Organizations

  • ASCO University Committee, ASCO (2011 - Present)
  • Professional Development Committee, ASCO (2011 - Present)
  • Oncology Training Programs Subcommittee, ASCO (2011 - Present)
  • Integrated Media and Technology Committee, ASCO (2011 - Present)
  • Excellence in Teaching Award Selection Committee, ASCO (2013 - Present)
  • Planning Committee Stanford Multidisciplinary Management of Cancers, Stanford University (2012 - Present)

Professional Education

  • Medical Education:Washington University in St Louis (2008) MO
  • Fellowship:Stanford Hospital and Clinics (2013) CA
  • Board Certification: Internal Medicine, American Board of Internal Medicine (2012)
  • Residency:Stanford Hospital and Clinics (2010) CA


Journal Articles

  • A multicenter phase II study of pazopanib in patients with advanced gastrointestinal stromal tumors (GIST) following failure of at least imatinib and sunitinib. Annals of oncology Ganjoo, K. N., Villalobos, V. M., Kamaya, A., Fisher, G. A., Butrynski, J. E., Morgan, J. A., Wagner, A. J., D'adamo, D., McMillan, A., Demetri, G. D., George, S. 2014; 25 (1): 236-240


    Advanced GISTs are incurable, but often treatable for years with tyrosine kinase inhibitors (TKIs). The majority of GISTs harbor an oncogenic activating mutation in KIT or PDGFRA. Inhibition of this activating mutation with TKIs most often leads to durable disease control for many patients. However, almost all patients develop resistance to these TKIs, typically due to the development of secondary mutations, heralding the need for new therapeutic options. We conducted a phase II study evaluating the efficacy and toxicity of pazopanib, a broad spectrum TKI inhibiting KIT, VEGFRs (-1, -2, and -3), and PDGFR (-α and-β) in patients with advanced GIST following failure of at least imatinib and sunitinib.Patients received pazopanib 800 mg orally once daily. All patients were assessed for efficacy with CT scans every 8 weeks (two cycles). Patients continued pazopanib until progression or unacceptable toxicity. The primary end point was the 24-week nonprogression [complete response+partial response+stable disease (SD)] rate (NPR) per RECIST 1.1. Secondary end points included PFS, OS, and toxicity.Between August 2011 and September 2012, a total of 25 patients were treated at two institutions. Median number of prior therapy was 3 (range 2-7). A total of 90 cycles of pazopanib were administered, with a median of two cycles (range 1 to 17+) per patient. Best response of SD at any time was observed in 12 (48%) patients. The NPR was 17% [95% confidence interval (CI) 4.5-37]. All but one patient discontinued protocol either due to PD (n = 19) or intolerance (n = 4). One patient with succinate dehydrogenase (SDH)-deficient GIST exhibited continuing disease control after 17 cycles. The median PFS for the entire cohort was 1.9 months (95% CI 1.6-5.2), and the median OS was 10.7 months (95% CI 3.9-NR).Pazopanib was reasonably well tolerated with no unexpected toxicities. Pazopanib as a single agent has marginal activity in unselected heavily pretreated patients with advanced GIST.

    View details for DOI 10.1093/annonc/mdt484

    View details for PubMedID 24356634

  • Identification of ovarian cancer driver genes by using module network integration of multi-omics data INTERFACE FOCUS Gevaert, O., Villalobos, V., Sikic, B. I., Plevritis, S. K. 2013; 3 (4)
  • Chest Wall Leiomyosarcoma After Breast-Conservative Therapy for Early-Stage Breast Cancer in a Young Woman With Li-Fraumeni Syndrome JOURNAL OF THE NATIONAL COMPREHENSIVE CANCER NETWORK Henry, E., Villalobos, V., Million, L., Jensen, K. C., West, R., Ganjoo, K., Lebensohn, A., Ford, J. M., Telli, M. L. 2012; 10 (8): 939-942


    Li-Fraumeni syndrome (LFS) is one of the most penetrant forms of familial cancer susceptibility syndromes, characterized by early age at tumor onset and a wide spectrum of malignant tumors. Identifying LFS in patients with cancer is clinically imperative because they have an increased sensitivity to ionizing radiation and are more likely to develop radiation-induced secondary malignancies. This case report describes a young woman whose initial presentation of LFS was early-onset breast cancer and whose treatment of this primary malignancy with breast conservation likely resulted in a secondary malignancy arising in her radiation field. As seen in this case, most breast cancers in patients with LFS exhibit a triple-positive phenotype (estrogen receptor-positive/progesterone receptor-positive/HER2-positive). Although this patient met classic LFS criteria based on age and personal and family history of cancer, the NCCN Clinical Practice Guidelines in Oncology for Genetic/Familial High-Risk Assessment: Breast and Ovarian Cancer endorse genetic screening for TP53 mutations in a subset of patients with early-onset breast cancer, even in the absence of a suggestive family history, because of the potential for de novo TP53 mutations.

    View details for Web of Science ID 000307494000004

    View details for PubMedID 22878818

  • Dual-Color Click Beetle Luciferase Heteroprotein Fragment Complementation Assays CHEMISTRY & BIOLOGY Villalobos, V., Naik, S., Bruinsma, M., Dothager, R. S., Pan, M., Samrakandi, M., Moss, B., Elhammali, A., Piwnica-Worms, D. 2010; 17 (9): 1018-1029


    Understanding the functional complexity of protein interactions requires mapping biomolecular complexes within the cellular environment over biologically relevant time scales. Herein, we describe a set of reversible multicolored heteroprotein complementation fragments based on various firefly and click beetle luciferases that utilize the same substrate, D-luciferin. Luciferase heteroprotein fragment complementation systems enabled dual-color quantification of two discrete pairs of interacting proteins simultaneously or two distinct proteins interacting with a third shared protein in live cells. Using real-time analysis of click beetle green and click beetle red luciferase heteroprotein fragment complementation applied to β-TrCP, an E3-ligase common to the regulation of both β-catenin and IκBα, GSK3β was identified as a candidate kinase regulating IκBα processing. These dual-color protein interaction switches may enable directed dynamic analysis of a variety of protein interactions in living cells.

    View details for DOI 10.1016/j.chembiol.2010.06.018

    View details for Web of Science ID 000283283200016

    View details for PubMedID 20851351

  • Rational Design of Novel Red-Shifted BRET Pairs: Platforms for Real-Time Single-Chain Protease Biosensors BIOTECHNOLOGY PROGRESS Gammon, S. T., Villalobos, V. A., Roshal, M., Samrakandi, M., Piwnica-Worms, D. 2009; 25 (2): 559-569


    Bioluminescence resonance energy transfer (BRET) systems to date have been dominated by use of blue-green Renilla luciferase (Rluc) as the energy donor. Although effective in many cases, the expense and unfavorable biochemical attributes of the substrate (phenylcoelenterazine) limit utility of Rluc-based BRET systems. Herein we report a series of novel BRET pairs based on luciferases that utilize D-luciferin, resulting in red-shifted photonic outputs, favorable biochemical attributes, and increased efficacy. We developed a modified Förster equation to predict optimal BRET luciferase donor-fluorophore pairs and identified tdTomato as the optimal red fluorophore acceptor for click beetle green luciferase (CBG). A prototypical single-chain protease biosensor, capable of reporting on executioner caspase activity in live cells and in real-time, was generated by inserting a DEVD linker between CBG and tdTomato and validated in vitro with recombinant caspases and in cellulo with apoptosis-sensitive and -resistant cell lines. High signal-to-noise ratios ( approximately 33) and Z' factors (0.85) were observed in live cell longitudinal studies, sufficient for high-throughput screening. Thus, we illustrate a general methodology for the rational design of new BRET systems and provide a novel single-chain BRET protease biosensor that is long lived, red-shifted, and utilizes D-luciferin.

    View details for DOI 10.1021/bp.144

    View details for Web of Science ID 000265572700033

    View details for PubMedID 19330851

  • Detection of protein-protein interactions in live cells and animals with split firefly luciferase protein fragment complementation. Methods in molecular biology (Clifton, N.J.) Villalobos, V., Naik, S., Piwnica-Worms, D. 2008; 439: 339-352


    Protein fragment complementation has emerged as a powerful tool for measuring protein-protein interactions in the context of live cells. The adaptation of this strategy for use with firefly luciferase now allows for the non-invasive, quantitative, real-time readout of protein interactions in lysates, live cells, and whole animals. Bioluminescence provides a robust imaging modality due to its extremely low background signal and large dynamic range. The split luciferase fusion constructs described here are inducible by addition of ligands, small molecules or drugs, in this example, rapamycin, and have been shown to work in vivo.

    View details for DOI 10.1007/978-1-59745-188-8_23

    View details for PubMedID 18370114

  • Current state of imaging protein-protein interactions in vivo with genetically encoded reporters ANNUAL REVIEW OF BIOMEDICAL ENGINEERING Villalobos, V., Naik, S., Piwnica-Worms, D. 2007; 9: 321-349


    Signaling pathways regulating proliferation, differentiation, and inflammation are commonly mediated through protein-protein interactions as well as reversible modification (e.g., phosphorylation) of proteins. To facilitate the study of regulated protein-protein interactions in cells and living animals, new imaging tools, many based on optical signals and capable of quantifying protein interactions in vivo, have advanced the study of induced protein interactions and their modification, as well as accelerated the rate of acquisition of these data. In particular, use of protein fragment complementation as a reporter strategy can accurately and rapidly dissect protein interactions with a variety of readouts, including absorbance, fluorescence, and bioluminescence. This review focuses on the development and validation of bioluminescent protein fragment complementation reporters that use either Renilla luciferase or firefly luciferase in vivo. Enhanced luciferase complementation provides a platform for near real-time detection and characterization of regulated and small-molecule-induced protein-protein interactions in intact cells and living animals and enables a wide range of novel applications in drug discovery, chemical genetics, and proteomics research.

    View details for DOI 10.1146/annurev.bioeng.9.060906.152044

    View details for Web of Science ID 000249337000011

    View details for PubMedID 17461729

  • Permeation peptide conjugates for in vivo molecular imaging applications MOLECULAR IMAGING Bullok, K. E., Gammon, S. T., Violini, S., Prantner, A. M., Villalobos, V. M., Sharma, V., Piwnica-Worms, D. 2006; 5 (1): 1-15


    Rapid and efficient delivery of imaging probes to the cell interior using permeation peptides has enabled novel applications in molecular imaging. Membrane permeant peptides based on the HIV-1 Tat basic domain sequence, GRKKRRQRRR, labeled with fluorophores and fluorescent proteins for optical imaging or with appropriate peptide-based motifs or macrocycles to chelate metals, such as technetium for nuclear scintigraphy and gadolinium for magnetic resonance imaging, have been synthesized. In addition, iron oxide complexes have been functionalized with the Tat basic domain peptides for magnetic resonance imaging applications. Herein we review current applications of permeation peptides in molecular imaging and factors influencing permeation peptide internalization. These diagnostic agents show concentrative cell accumulation and rapid kinetics and display cytosolic and focal nuclear accumulation in human cells. Combining methods, dual-labeled permeation peptides incorporating fluorescein maleimide and chelated technetium have allowed for both qualitative and quantitative analysis of cellular uptake. Imaging studies in mice following intravenous administration of prototypic diagnostic permeation peptides show rapid whole-body distribution allowing for various molecular imaging applications. Strategies to develop permeation peptides into molecular imaging probes have included incorporation of targeting motifs such as molecular beacons or protease cleavable domains that enable selective retention, activatable fluorescence, or targeted transduction. These novel permeation peptide conjugates maintain rapid translocation across cell membranes into intracellular compartments and have the potential for targeted in vivo applications in molecular imaging and combination therapy.

    View details for DOI 10.2310/7290.2006.00001

    View details for Web of Science ID 000239086900001

    View details for PubMedID 16779965

  • Quantitative analysis of permeation peptide complexes labeled with technetium-99m: Chiral and sequence-specific effects on net cell uptake BIOCONJUGATE CHEMISTRY Gammon, S. T., Villalobos, V. M., Prior, J. L., Sharma, V., Piwnica-Worms, D. 2003; 14 (2): 368-376


    This study investigated sequence-specific cell uptake characteristics of Tat basic domain and related permeation peptides with an emphasis on residue chirality, length, and modified side chains. Effects on cell permeation of defined basic domain sequences within a library of 42 different peptides were evaluated using transport of radiolabeled peptides into human Jurkat leukemia cells. All other factors being equal, when the chirality of the peptide sequence was changed from l to d, uptake values increased up to 13-fold. Control experiments showed that the quantitative difference in uptake could not be attributed to increased decomposition of an l- versus a d-peptide by cellular or serum proteases. Furthermore, length, sequence, and type of chelation domain impacted peptide uptake into cells. The highest level of uptake was found with the following peptides: (23) d-Tat-Orn [Ac-rkkrr-orn-rrr-AHA-kgc-amide] and (33) d-poly-Arg(9) [Ac-rrrrrrrrr-AHA-kgc-amide]. The best of these peptide sequences could be employed as in vivo imaging and drug delivery agents to translocate substrates into cells.

    View details for DOI 10.1021/bc0256291

    View details for Web of Science ID 000181718700015

    View details for PubMedID 12643747

  • Synthesis and biological evaluation of aryl azide derivatives of combretastatin A-4 as molecular probes for tubulin BIOORGANIC & MEDICINAL CHEMISTRY Pinney, K. G., Mejia, M. P., Villalobos, V. M., Rosenquist, B. E., Pettit, G. R., Verdier-Pinard, P., Hamel, E. 2000; 8 (10): 2417-2425


    Two new aryl azides, (Z)-1-(3'-azido-4'-methoxyphenyl)-2-(3",4",5"-trimethoxyphenyl)ethene 9 and (Z)-1-(4'-azido-3'-methoxyphenyl)-2-(3",4",5"-trimethoxyphenyl)ethene 5, modeled after the potent antitumor, antimitotic agent combretastatin A-4 (CA-4), have been prepared by chemical synthesis as potentially useful photoaffinity labeling reagents for the colchicine site on beta-tubulin. Aryl azide 9, in which the 3'-hydroxyl group of CA-4 is replaced by an azido moiety, demonstrates excellent in vitro cytotoxicity against human cancer cell lines (NCI 60 cell line panel, average GI50 = 4.07 x 10(-8) M) and potent inhibition of tubulin polymerization (IC50 = 1.4+/-0.1 microM). The 4'-azido analogue 5 has lower activity (NCI 60 cell line panel, average GI50 = 2.28 x 10(-6) M, and IC50 = 5.2+/-0.2 microM for inhibition of tubulin polymerization), suggesting the importance of the 4'-methoxy moiety for interaction with the colchicine binding site on tubulin. These CA-4 aryl azide analogues also inhibit binding of colchicine to tubulin, as does the parent CA-4, and therefore these compounds are excellent candidates for photoaffinity labeling studies.

    View details for Web of Science ID 000089624800004

    View details for PubMedID 11058036

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