RETROSPECTIVE

A review of Judah Folkman’s remarkable achievements in biomedicine

Yihai Cao*† and Robert Langer‡ *Department of Microbiology and Tumor and Cell Biology, Karolinska Institute, 171 77 Stockholm, ; and ‡Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

n January 14th of this year, the biomedical research com- munity lost Judah Folkman, O the father of research. Folkman’s warm and humble personality, inspirational teaching, un- limited creativity, and vast clinical expe- rience have been described elsewhere (1). Here, we focus on Folkman’s in- effaceable scientific achievements in angiogenesis research, which revolution- ized biomedical research and clinical drug development. Folkman founded an entirely new field of basic and clinical research and discovered a previously unknown family of angiogenesis regulatory molecules. He showed experimentally how these molecules provide a fundamental mech- anism that controls the growth of virtually all tumors. He showed that expansion of tissue mass, whether Judah Folkman. (Image courtesy of Jon Chase/Harvard News Office.) neoplastic or nonneoplastic, critically depends on continuous endothelial rep- This 1971 report was the first to intro- the cornea, a methodology that was crit- lication and neovascularization. duce the concept of a novel form of tu- ical for proof of angiogenic bioactivity Those discoveries paved new avenues mor dormancy caused by blockage of of a given molecule in vivo (5). His re- for the development of a new class of angiogenesis. In that paper, Folkman search also identified the existence of a drugs, angiogenesis inhibitors, which have also introduced the concept of ‘‘anti- family of angiogenic peptides (6), and already provided novel therapies for hu- angiogenesis’’ as a potential novel anti- he showed that removing an angiogenic man cancer and age-related macular cancer therapy. stimulus leads to regression of degeneration. Folkman wrote in 1971, ‘‘If a tumor neovascularization (7). Professor Donald S. Coffey of The could be held indefinitely in the non- Folkman’s 1971 supposition initiated a Johns Hopkins University (Baltimore, vascularized dormant state...itispossi- field of angiogenesis research that has MD) recently said about professor Folk- ble that metastases will not arise.’’ He produced Ͼ35,000 reports on angiogen- man: ‘‘Few ever get to see their lifetime also predicted that a therapeutic agent esis in peer-reviewed journals, and Ͼ70 thoughts, insights, creativity, and long could be made against a putative tumor- monographs and books. effort explode across science and into derived angiogenic factor. He con- This single hypothesis has been re- the clinics as [his] many contributions cluded, ‘‘It has not been appreciated markably fruitful. During the subse- have.’’ that the population of tumor cells and quent 37 years, it led to an almost un- Fortunately, we are among those few the population of capillary endothelial interrupted series of new findings and who had the privilege to see and work cells within a neoplasm may constitute a discoveries, not only in the Folkman with Dr. Folkman, who changed not highly integrated ecosystem. In this eco- laboratory but in hundreds of laborato- only our lives but those of millions of system, the mitotic index of the two cell ries worldwide. others. populations may depend on each other.’’ Folkman followed his 1971 hypothesis Molecular Basis of the Discovery Early Evidence of Tumor Angiogenesis with supporting evidence indicating that Folkman showed that human and ani- In 1971, Folkman proposed a hypothesis tumors secreted diffusible endothelial mal tumors produce a defined set of that tumor growth is angiogenesis- mitogens in vivo (3), which could induce proangiogenic and antiangiogenic pro- dependent (2). In that report, Folkman angiogenesis in neighboring micro- teins. When the expression, secretion, or showed preliminary evidence that tu- vessels. He and his colleagues then generation of these proteins is in equi- mors could not enlarge beyond millime- showed that tumor growth could be in- librium so that their proangiogenic and ter diameters without recruiting new hibited by blocking angiogenesis, and capillary blood vessels (microvessels). they proved that diffusible angiogenesis His hypothesis stated that tumors se- inhibitory factors existed (4). Author contributions: Y.C. and R.L. wrote the paper. creted a diffusible substance that could Folkman and colleagues also reported †To whom correspondence should be addressed. E-mail: stimulate endothelial cell proliferation in that macromolecules and proteins can [email protected]. host capillary blood vessels. be released from polymers implanted in © 2008 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0806582105 PNAS ͉ September 9, 2008 ͉ vol. 105 ͉ no. 36 ͉ 13203–13205 Downloaded by guest on October 2, 2021 antiangiogenic activities are balanced, or naturally occurring angiogenic and anti- tional imaging methods. This platelet an- oppose each other, tumor mass cannot angiogenic molecules existed, a new bi- giogenesis proteome is now being studied expand beyond the limited distance that ology of microvascular endothelial cells in clinical trials at Harvard (Cambridge, oxygen can diffuse from the nearest was emerging, a new form of tumor dor- MA) in collaboration with St. Jude’s Can- open capillary blood vessel (Ͻ250 mi- mancy was caused by angiogenic block- cer Center (Memphis, TN) for ultra-early crons). As a result of these metabolic ade, and tumor growth and metastases detection of recurrent cancer years before restrictions, the development of most were angiogenesis-dependent. it is symptomatic or can be anatomically human cancer is usually arrested at a Around that time, Folkman and his located. microscopic, dormant mass of 1–2 mm3 associates began experiments that laid Folkman and his associates (20) also or less. This early stage is also called in the groundwork for translation of these showed that infants with a life- situ cancer. molecules and mechanisms to clinical threatening condition called hemangio- Many humans harbor these micro- application. He demonstrated that the matosis, which results from an abnormal scopic tumors in various organs through- rapid growth of metastases after surgical proliferation of blood vessels in any vas- out life (8), and the vast majority of removal or radiation therapy of certain cularized tissue, could be successfully such tumors never expand further. They primary tumors, a phenomenon previ- treated by antiangiogenic therapy, and remain nonangiogenic, dormant, and ously called ‘‘concomitant immunity’’ or that experimental endometriosis could harmless (9). They may contain up to ‘‘concomitant resistance’’ is in fact also be inhibited by antiangiogenic Ϸ 1 million tumor cells that are prolifer- caused by withdrawal of angiostasis (14). therapy (21). ating and undergoing apoptosis, but the He also reported that human leukemia A Beautiful Approach microscopic tumors do not invade lo- is angiogenic, and he showed in animal cally and do not metastasize. models that leukemia is angiogenesis- Folkman hypothesized that certain In the rare event that a shift in the dependent (15). Before Folkman and col- nonneoplastic diseases such as pso- balance of proangiogenic and antiangio- leagues’ reports, physicians believed that riasis, rheumatoid arthritis, and ocu- genic proteins occurs, so that the total because leukemic cells circulated in blood, lar neovascularization could also be output of these proteins renders the tu- the growth of leukemia cells did not need angiogenesis-dependent. He suggested mor mass proangiogenic, the tumor is neovascularization. Folkman showed that that angiogenesis inhibitors developed then enabled to ‘‘switch to an angio- neovascularization in the bone marrow for cancer therapy could possibly be genic phenotype’’ (10). As first eluci- used to treat nonneoplastic angiogene- was critical for expansion of leukemic dated by Folkman, the tumor itself sis-dependent diseases (22). mass in the marrow. expresses and secretes this excess of A beautiful example of this translational proangiogenic proteins or, alternatively, approach is the concerted effort launched it enzymatically mobilizes proangiogenic in 1992 by the Folkman laboratory with peptides from host proteins in the circu- “Angiogenesis research their collaborators to find the molecular lation or the supportive framework of target for choroidal and retinal neovascu- an organ or gland (stroma). will probably change the larization in humans. This target was The angiogenic switch can also be mod- face of in the unknown before the mid-1990s. ulated by host cells in the stroma or the Ophthalmologists had previously used circulation. After the angiogenic switch, next decades.” the term ‘‘X-factor’’ to indicate a putative both benign and malignant tumors can diffusible molecule that mediated neovas- expand their tumor mass. Malignant an- cular age-related macular degeneration. In giogenic tumors, however, invade locally, the mid-1990s, Folkman and colleagues metastasize to remote sites, and are po- Folkman and colleagues (16) also published a series of papers (e.g., see ref. tentially lethal. In the absence of the an- showed for the first time that certain con- 23) demonstrating that vascular endothe- giogenic switch, even tumors that are ventional cytotoxic chemotherapeutic lial growth factor (VEGF) is a critical malignant by all other criteria may be agents, when administered frequently and mediator of retinal neovascularization, so harmless to the host (11). at low doses, are endothelial-dependent. it would therefore be an appropriate tar- Beginning in 1980, Folkman was the He named this ‘‘antiangiogenic chemo- get for antiangiogenic therapy of age- first to completely purify angiogenesis reg- therapy.’’ Others (17) subsequently called related neovascular macular degeneration. ulatory molecules. This identification of it ‘‘metronomic therapy.’’ Those studies led directly to the clinical angiogenesis regulatory molecules became His work showed that tumors that had testing and eventual U.S. Food and Drug possible because of three novel bioassays become completely resistant to conven- Administrative (FDA) approval of Pe- that he had developed in the 1970s: tional cytotoxic therapy administered at gaptanib (Macugen) which is an aptamer cloned capillary endothelial cells in vitro maximum tolerated doses followed by of VEGF, and ranibizumub (Lucentis), (12); neovascularization induced in the off-therapy intervals could, in contrast, which is an anti-VEGF antibody. rabbit and mouse corneas by angiogenic be significantly inhibited or permanently These inhibitors are also approved in proteins undergoing sustained release regressed by antiangiogenic many countries, including those in the from implanted polymers (4, 5); and the chemotherapy (18). European Union, India, and Switzer- ex vivo chicken embryo chorioallantoic Along with his collaborators, Folk- land. With Lucentis, nearly 40% of membrane (13). Today, at least 28 endoge- man showed that platelets sequester patients who are blind (visual acuity nous angiogenesis inhibitory molecules, angiogenesis regulatory proteins and of 20/300) regain their eyesight within and Ͼ10 proangiogenic molecules, have segregate them into two sets of alpha a few months of initiation of local been discovered in the body. Many others granules, according to proangiogenic therapy (24). are synthetic molecules. and antiangiogenic bioactivity (19). Their analysis of the ‘‘platelet angiogen- Clinical Drug Delivery Preclinical Applications of Angiogenesis esis proteome’’ in tumor-bearing mice re- Folkman’s discovery that tumor growth is By the late 1980s, the scientific commu- vealed that the presence of human tumors angiogenesis-dependent led to a profound nity had begun to accept some of the can be detected at a microscopic size that paradigm shift in cancer therapy and can- general principles of tumor angiogenesis: is below the resolution of any conven- cer biology. Before his 1971 hypothesis,

13204 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0806582105 Cao and Langer Downloaded by guest on October 2, 2021 the cancer cell per se was the only target The FDA subsequently approved bev- In 2006, Ϸ1.25 million patients re- of cancer therapies. Now it is accepted acizumab (Avastin) as a first-line treat- ceived a prescription for one of these that the microvascular endothelial cell ment for patients with metastatic antiangiogenic drugs, and during the recruited by tumors is a second target. colorectal cancer. Three months later, first 11 months of 2007, Ͼ1.5 million Before its FDA approval in 2003 for Andrew C. von Eschenbach, the director patients received prescriptions for anti- multiple myeloma, bortezomib (Vel- of the National Cancer Institute, and angiogenic therapy (from a search of cade) was reported to have potent anti- Allen M. Spiegel, the director of the worldwide databases by ‘‘info2go,’’ angiogenic activity (25). Since 2003, 10 National Institute of Diabetes and Di- Thomas Pharma, Chemical Market Re- drugs that are pure angiogenesis inhibi- gestive and Kidney Diseases, wrote that porter and Business Communications). tors or that have potent antiangiogenic ‘‘the approval marked the arrival of an At the end of 2007, Ϸ23 drugs with an- activity have been approved by the FDA Ͼ intervention in which the primary mech- tiangiogenic activity were in phase III and similar regulatory agencies in 40 Ͼ other countries. Eight of these are being anism of action is angiogenesis inhibi- clinical trials, and 30 were in phase II used to treat cancer, and two are used tion. We now can unequivocally say that clinical trials. for the treatment of age-related macular angiogenesis is not only a critical fac- In December 2005, Nature published a degeneration (26). tor for cancer, but for a host of other major review of the field of angiogenesis In February 2004, FDA Commis- diseases.’’ research and predicted that “angiogene- sioner Mark McClellan announced that, Significant increases in survival have sis research will probably change the in addition to surgery, radiotherapy, and already been reported for patients face of medicine in the next decades” chemotherapy, ‘‘antiangiogenic therapy with colon cancer and lung cancer with more than 500 million people can now be considered the fourth mo- treated with the worldwide predicted to benefit from dality of cancer treatment.’’ . pro- or antiangiogenesis treatments (27).

1. Klagsbrun M, Moses MA (2008) Obituary: M. Judah 11. O’Reilly MS, et al. (1994) A novel angiogenesis inhibitor 19. Italiano JE, Jr, et al. (2008) Angiogenesis is regulated by Folkman (1933–2008). Nature 451:781. that mediates the supression of a metastases by a Lewis a novel mechanism: Pro- and antiangiogenic proteins 2. Folkman J (1971) Tumor angiogenesis: Therapeutic im- lung carcinoma. Cell 79:315–328. are organized into separate platelet alpha granules plications. N Engl J Med 285:1182–1186. 12. Folkman J, Haudenschild CC, Zetter BR (1979) Long- and differentially released. Blood 111:1227–1233. 3. Cavallo T, Sade R, Folkman J, Cotran RS (1972) Tumor term culture of capillary endothelial cells. Proc Natl 20. Ezekowitz A, Mulliken J, Folkman J (1991) Interferon angiogenesis: Rapid induction of endothelial mitoses Acad Sci USA 76:5217–5221. alpha therapy of haemangiomas in newborns and in- demonstrated by autoradiography. J Cell Biol 54:408– 13. Ausprunk DH, Knighton DR, Folkman J (1974) Differ- fants. Br J Hematol 79(Suppl 1):67–68. 420. entiation of vascular endothelium in the chick cho- 21. Becker CM, et al. (2006) Short synthetic 4. Langer R, Folkman J (1976) Polymers for the sustained rioallantois: A structural and autoradiographic study. peptides inhibit endothelial migration in vitro and release of proteins and other macromolecules. Nature Dev Biol 38:237–248. endometriosis in a mouse model. Fertil Steril 85:71–77. 263:797–800. 14. Cao Y, et al. (1998) Expression of cDNA in a 22. Folkman J (1972) Angiogenesis in psoriasis: Therapeu- 5. Langer R, Brem H, Falterman K, Klein M, Folkman J murine fibrosarcoma suppresses primary tumor tic implications. J Invest Dermatol 59:40–43. (1976) Isolation of a cartilage factor which inhibits growth and produces long-term dormancy of metasta- 23. Adamis AP, et al. (1996) Inhibition of vascular endo- tumor neovascularization. Science 193:70–72. ses. J Clin Invest 101:1055–1063. thelial growth factor prevents retinal ischemia- 6. Folkman J, Klagsbrun M (1987) Angiogenic factors. 15. Perez-Atayde AR, et al. (1997) Spectrum of tumor an- associated neovascularization in a nonhuman primate. Science 235:442–447. giogenesis in the bone marrow of children with acute Arch Ophthalmol 114:66–71. 7. Ausprunk DH, Falterman K, Folkman J (1978) The se- lymphoblastic leukemia. Am J Pathol 150:815–821. 24. Rosenfeld PJ, et al. (2006) for neovascular quence of events in the regression of corneal capillar- ies. Lab Invest 38:284–294. 16. Browder T, et al. (2000) Antiangiogenic scheduling of age-related macular degeneration. N Engl J Med 8. Black WC, Welch HG (1993) Advances in diagnostic chemotherapy improves efficacy against experimental 355:1419–1431. imaging and overestimations of disease prevalence drug-resistant cancer. Cancer Res 60:1878–1886. 25. Sunwoo JB, et al. (2001) Novel proteasome inhibitor ␬ and the benefits of therapy. N Engl J Med 328:1237– 17. Gately S, Kerbel R (2001) Antiangiogenic scheduling PS-341 inhibits activation of nuclear factor- B, cell sur- 1243. of lower-dose cancer chemotherapy. Cancer J 7:427– vival, tumor growth, and angiogenesis in squamous cell 9. Folkman J, Kalluri R (2004) Cancer without disease. 436. carcinoma. Clin Cancer Res 7:1419–1428. Nature 427:787. 18. Kieran MW, et al. (2005) A feasibility trial of antuan- 26. Folkman J (2007) Angiogenesis: An organizing princi- 10. Almog N, et al. (2006) Prolonged dormancy of human giogenic chemotherapy in pediatric patients with re- ple for drug discovery. Nat Rev Drug Discov 6:273–286. liposarcoma is associated with impaired tumor angio- current or progressive cancer. J Pediatr Hematol Oncol 27. Carmeliet P (2005) Angiogenesis in life, disease, and genesis. FASEB J 20:947–949. 27:573–581. medicine. Nature 438:932–936.

Cao and Langer PNAS ͉ September 9, 2008 ͉ vol. 105 ͉ no. 36 ͉ 13205 Downloaded by guest on October 2, 2021