New anti-lung-cancer antibody cluster 12 reacts with human folate receptors present on adenocarcinoma

Folate receptor alpha as a tumor target in epithelial ovarian cancer

OBJECTIVES

Folate receptor alpha (FRalpha) is a folate-binding protein overexpressed on ovarian and several other epithelial malignancies that can be used as a target for imaging and therapeutic strategies. The goal of this study is to improve historical data that lack specific information about FRalpha expression in rare histological subtypes, primary disease versus metastatic foci, and recurrent disease.

METHODS

FRalpha expression was analyzed by immunohistochemistry on 186 primary and 27 recurrent ovarian tumors, including 24 pairs of samples obtained from the same individuals at diagnosis and at secondary debulking surgery. For 20 of the 186 primaries, simultaneous metastatic foci were also analyzed. FRalpha staining was analyzed in light of disease morphology, stage, grade, debulking status, and time from diagnosis to recurrence and death.

RESULTS

FRalpha expression was apparent in 134 of 186 (72%) primary and 22 of 27 (81.5%) recurrent ovarian tumors. In 21 of 24 (87.5%) matched specimens, recurrent tumors reflected the FRalpha status detected at diagnosis. Metastatic foci were similar to primary tumors in FRalpha staining. FRalpha status was not associated with time to recurrence or overall survival in either univariate or multivariable analyses.

CONCLUSION

FRalpha expression occurs frequently, especially in the common high-grade, high-stage serous tumors that are most likely to recur. New findings from this study show that FRalpha expression is maintained on metastatic foci and recurrent tumors, suggesting that novel folate-targeted therapies may hold promise for the majority of women with either newly diagnosed or recurrent ovarian cancer.

Drug Delivery Devices and Targeting Agents for Platinum(II) Anticancer Complexes

An ideal platinum-based delivery device would be one that selectively targets cancerous cells, can be systemically delivered, and is non-toxic to normal cells. It would be beneficial to provide drug delivery devices for platinum-based anticancer agents that exhibit high drug transport capacity, good water solubility, stability during storage, reduced toxicity, and enhanced anticancer activity in vivo. However, the challenges for developing drug delivery devices include carrier stability in vivo, the method by which extracellular or intracellular drug release is achieved, overcoming the various mechanisms of cell resistance to drugs, controlled drug release to cancer cells, and platinum drug bioavailability. There are many potential candidates under investigation including cucurbit[n]urils, cyclodextrins, calix[n]arenes, and dendrimers, with the most promising being those that are synthetically adaptable enough to attach to targeting agents.

Folate receptor overexpression is associated with poor outcome in breast cancer

The high affinity folate receptor is a membrane-associated glycoprotein that is preferentially expressed in cancers of epithelial origin and rarely expressed in normal cells. We examined its expression pattern in breast cancer, utilizing a tissue microarray containing samples from 63 invasive breast cancers from women with divergent clinical outcomes. Thirty-three women comprised the poor outcome group with a median time to recurrence of 1.9 years. Thirty women, the good outcome group, were free of recurrence for a minimum of 7 years after diagnosis. The intensity of folate receptor staining was strongly correlated with outcome. There were two summary categories of staining intensity: weak (n = 42) or strong (n = 21). In the strong staining group, 17 of 21 women (81%) have recurred and their median survival is 2.4 years. In the weak staining group, 16 of 42 women (38%) have recurred. Their median survival is not estimable. After adjustment for tumor size, nodal status, ER status, adjuvant therapy, histology and tumor grade, strong staining for the folate receptor remained significantly associated with poor outcome, p < 0.001. Our work requires validation in a larger cohort, but supports the possibility of using folate receptor-targeted approaches in the management of breast cancer.

Chemical modification of therapeutic drugs or drug vector systems to achieve targeted therapy: looking for the grail

Most therapeutic drugs distribute to the whole body, which results in general toxicity and poor acceptance of the treatments by patients. The targeted delivery of chemotherapeutics to defined cells, either stromal or cancer cells in cancer lesions, or defined inflammatory cells in immunological disorders, is one of the main challenges and a very active field of research in the development of treatment strategies to minimize side-effects of drugs. Disease-associated cells express molecules, including proteases, receptors, or adhesion molecules, that are different or differently expressed than their normal counterparts. Therefore one goal in the field of targeted therapies is to develop chemically derivatized drugs or drug vectors able to target defined cells via specific recognition mechanisms and also able to overcome biological barriers. This article will review the approaches which have been explored to achieve these goals and will discuss in more detail three examples (i) the use of nanostructures to take advantage of increased vascular permeability in some human diseases, (ii) the targeting of therapeutic drugs to an organ, the brain, protected against foreign molecules by the blood-brain barrier, and (iii) the use of the folate receptor to target either tumor cells or activated macrophages.

Design, synthesis, and biological evaluation of folic acid targeted tetraphenylporphyrin as novel photosensitizers for selective photodynamic therapy

Photodynamic therapy (PDT) is a cancer treatment involving systemic administration of a tumor-localizing photosensitizer; this, when activated by the appropriate light wavelength, interacts with molecular oxygen to form a toxic, short-lived species known as singlet oxygen, which is thought to mediate cellular death. Targeted PDT offers the opportunity of enhancing photodynamic efficiency by directly targeting diseased cells and tissues. Two new conjugates of three components, folic acid/hexane-1,6-diamine/4-carboxyphenylporphyrine 1 and folic acid/2,2'-(ethylenedioxy)-bis-ethylamine/4-carboxyphenylporphyrine 2 were synthesized. The conjugates were characterized by 1H NMR, MALDI, UV-visible spectroscopy, and fluorescence quantum yield. The targeted delivery of these photoactive compounds to KB nasopharyngeal cell line, which is one of the numerous tumor cell types that overexpress folate receptors was studied. It was found that after 24 h incubation, conjugates 1 and 2 cellular uptake was on average 7-fold higher than tetraphenylporphyrin (TPP) used as reference and that 1 and 2 cellular uptake kinetics increased steadily over the 24 h period, suggesting an active transport via receptor-mediated endocytosis. In corresponding results, conjugates 1 and 2 accumulation displayed a reduction of 70% in the presence of a competitive concentration of folic acid. Survival measurements demonstrated that KB cells were significantly more sensitive to conjugated porphyrins-mediated PDT. Under the same experimental conditions and the same photosensitizer concentration, TPP displayed no photocytotoxicity while conjugates 1 and 2 showed photodynamic activity with light dose values yielding 50% growth inhibition of 22.6 and 6.7 J/cm2, respectively.

Statistical approach for detection and localization of a fluorescing mouse tumor in Intralipid

We present a method for detecting and localizing a fluorescing tumor obscured underneath several millimeters of a multiply scattering, homogeneous medium from fluorescence measurements made above the surface. Using a statistical model of the measurement system, we develop approaches for detection by use of a binary hypothesis testing approach and localization by use of maximum-likelihood estimation. We also compute the probability of tumor detection and the Cramér-Rao lower bound for the localization estimate error, which are performance metrics that could potentially be optimized in an experimental design. We validate the methods in an experimental study involving an excised mouse tumor tagged with a new folate-indocyanine dye and obscured under a tissue-simulating lipid suspension.

Folate receptor expression in carcinomas and normal tissues determined by a quantitative radioligand binding assay

The folate receptor (FR) is a valuable therapeutic target that is highly expressed on a variety of cancers. The current development of folate-targeted cancer therapies has created the need for quantitating functional FRs in clinical specimens. In this article, we report on the creation of a highly sensitive radioactive binding method for quantitatively measuring FR expression in frozen tissue homogenates. Expression was positive in approximately 89% of human ovarian carcinomas but was negligible in both mucinous ovarian carcinomas and normal ovary. Expression was also significant in carcinomas of the kidney, endometrium, lung, breast, bladder, and pancreas. Normal tissues from humans and six different laboratory species were also analyzed; surprisingly, some interspecies variability in FR expression (especially in kidney, spleen, and lung tissue) was found. Interestingly, normal human lung tissue displayed high expression levels, whereas expression in normal lung of the other species was negligible. However, considering that folate-drug conjugates fail to accumulate in the lungs of patients, the consequence of this finding was not considered to be of clinical concern. Overall, this new methodology is reliable for determining functional FR expression levels in tissues, and it could possibly be a useful clinical test to determine patient candidacy for FR-targeted therapeutics.

Uptake of folate-conjugated albumin nanoparticles to the SKOV3 cells

The bovine serum albumin nanoparticles (BSANPs) were prepared by a coacervation method and chemical cross-linking with glutaraldehyde. Furthermore, the BSANPs were reacted with the activated folic acid to conjugate folate via amino groups of the BSANPs, to improve their intracellular uptake to target cells. The nanoparticles were apparently spherical with diameters less than 150 nm and their average diameter was 70 nm by a transmission electron microscope (TEM) and a laser light scattering particle analyzer, respectively. An extent of folate conjugation with the BSANPs was 169 micromol/g BSA by spectrophotometric analysis. Cell uptake studies were carried out in SKOV3 cells (human ovarian cancer cell line) using fluorescein isothiocyanate labeled nanoparticles. The extent of BSANPs taken up by the cells was measured with a fluorescence spectrophotometer. The nanoparticles were taken up to the cells and levels of binding and uptake were increased with the time of incubation until 4 h. The levels of folate-conjugated BSANPs were higher than those of BSANPs and saturable. The association of folate-conjugated BSANPs to SKOV3 cells was inhibited by an excess amount of folic acid, suggesting that the binding and/or uptake were mediated by the folate receptor. These results implied that the folate-conjugated BSANPs might be useful as a drug carrier system to deliver drugs into the cells expressing folate receptor.

Increased uptake of folate conjugates by activated macrophages in experimental hyperlipemia

In the pathogenesis of atherosclerosis, macrophages become activated and play a crucial role in plaque formation. Activated synovial macrophages have recently been shown to express receptors for folic acid. We have determined whether activated macrophages also over-express folate receptor (FR) in atherosclerosis. Most normal cells express little or no FR, and, if FR is present on activated macrophages, folate-linked compounds and drugs could be selectively targeted to those cells that do express FR. To evaluate the FR on macrophages of atherosclerotic animals, golden Syrian hamsters were maintained on a hyperlipidemic diet until extensive vascular lesions had developed. Uptake of folic acid conjugated to fluorescent tags was then examined in tissue fragments from lesion-prone areas, and peritoneal activated macrophages were harvested from the same animals. Spectrofluorimetric and fluorescence microscopic analyses showed a significantly greater uptake of folate-conjugates by peritoneal macrophages of hyperlipidemic hamsters compared with those of hamsters fed a normal or folate-deficient diet. Systemically administered folate-fluorescent conjugates were found to accumulate as bright spots in protrusions of atherosclerotic plaques populated by macrophages, whereas a low level of fluorescence was detected uniformly dispersed across the lesion. The uptake of the folate conjugate by U937 macrophage cells grown in a high-lipid culture medium was significantly higher than in controls. Our data thus indicate that hyperlipidemic conditions induce an increased uptake of folate attributable to the over-expression of FRs on activated macrophages. This increase in FR expression can be exploited to deliver folate-linked compounds selectively to atherosclerotic lesions.

Folate receptor alpha and caveolae are not required for Ebola virus glycoprotein-mediated viral infection

Folate receptor alpha (FRalpha) has been described as a factor involved in mediating Ebola virus entry into cells (6). Furthermore, it was suggested that interaction with FRalpha results in internalization and subsequent viral ingress into the cytoplasm via caveolae (9). Descriptions of cellular receptors for Ebola virus and its entry mechanisms are of fundamental importance, particularly with the advent of vectors bearing Ebola virus glycoprotein (GP) being utilized for gene transfer into cell types such as airway epithelial cells. Thus, the ability of FRalpha to mediate efficient entry of viral pseudotypes carrying GP was investigated. We identified cell lines and primary cell types such as macrophages that were readily infected by GP pseudotypes despite lacking detectable surface FRalpha, indicating that this receptor is not essential for Ebola virus infection. Furthermore, we find that T-cell lines stably expressing FRalpha are not infectible, suggesting that FRalpha is also not sufficient to mediate entry. T-cell lines lack caveolae, the predominant route of FRalpha-mediated folate metabolism. However, the coexpression of FRalpha with caveolin-1, the major structural protein of caveolae, was not able to rescue infectivity in a T-cell line. In addition, other cell types lacking caveolae are fully infectible by GP pseudotypes. Finally, a panel of ligands to and soluble analogues of FRalpha were unable to inhibit infection on a range of cell lines, questioning the role of FRalpha as an important factor for Ebola virus entry.

Optical imaging of metastatic tumors using a folate-targeted fluorescent probe

We describe the use of a tumor targeting ligand, the vitamin folic acid, to deliver an attached fluorescent probe to both primary and metastatic tumors overexpressing the folate receptor. Upon laser excitation, derived images of normal tissues generally show little or no fluorescence, whereas images of folate receptor-expressing tumors display bright fluorescence that can be easily distinguished from adjacent normal tissue. Furthermore, metastatic tumor loci of submillimeter size can also be visualized without the aid of image processing or enhancement. The sharp distinction between tumor and normal tissues provided by this technique could find application in the localization and resection of tumor tissue during surgery or in the enhanced endoscopic detection and staging of cancers.

Folate receptor-targeted liposomes as vectors for therapeutic agents

The folate receptor is a cell surface protein that has recently been identified as a tumor marker, due to its differential overexpression in several malignancies. Current research indicates that folate can be covalently attached to the surface of liposomes to mediate their selective internalization by tumor cells through the folate receptor-mediated endocytic pathway. Optimized liposome formulations, characterized by improvements in drug loading, extended residence times in the circulation and improved drug release, have been developed to improve the biodistribution of therapeutic molecules. Theoretically, folate receptor-targeting can be combined with liposome encapsulation to synergistically affect disease outcome by enhancing the delivery of chemotherapeutic agents to neoplastic cells, while reducing systemic toxicities to normal tissues. The purpose of this chapter is to characterize the components of folate receptor-targeted liposomes, and summarize their applications in gene and drug delivery.

Lentivirus vectors pseudotyped with filoviral envelope glycoproteins transduce airway epithelia from the apical surface independently of folate receptor alpha

The practical application of gene therapy as a treatment for cystic fibrosis is limited by poor gene transfer efficiency with vectors applied to the apical surface of airway epithelia. Recently, folate receptor alpha (FR alpha), a glycosylphosphatidylinositol-linked surface protein, was reported to be a cellular receptor for the filoviruses. We found that polarized human airway epithelia expressed abundant FR alpha on their apical surface. In an attempt to target these apical receptors, we pseudotyped feline immunodeficiency virus (FIV)-based vectors by using envelope glycoproteins (GPs) from the filoviruses Marburg virus and Ebola virus. Importantly, primary cultures of well-differentiated human airway epithelia were transduced when filovirus GP-pseudotyped FIV was applied to the apical surface. Furthermore, by deleting a heavily O-glycosylated extracellular domain of the Ebola GP, we improved the titer of concentrated vector severalfold. To investigate the folate receptor dependence of gene transfer with the filovirus pseudotypes, we compared gene transfer efficiency in immortalized airway epithelium cell lines and primary cultures. By utilizing phosphatidylinositol-specific phospholipase C (PI-PLC) treatment and FR alpha-blocking antibodies, we demonstrated FR alpha-dependent and -independent entry by filovirus glycoprotein-pseudotyped FIV-based vectors in airway epithelia. Of particular interest, entry independent of FR alpha was observed in primary cultures of human airway epithelia. Understanding viral vector binding and entry pathways is fundamental for developing cystic fibrosis gene therapy applications.

Expression profiles of non-small cell lung cancers on cDNA microarrays: identification of genes for prediction of lymph-node metastasis and sensitivity to anti-cancer drugs

To investigate genes involved in pulmonary carcinogenesis and those related to sensitivity of nonsmall cell lung cancers (NSCLCs) to therapeutic drugs, we performed cDNA microarray analysis of 37 NSCLCs after laser-capture microdissection of cancer cells from primary tumors. A clustering algorithm applied to the expression data easily distinguished two major histological types of non-small cell lung cancer, adenocarcinoma and squamous cell carcinoma. Subsequent analysis of the 18 adenocarcinomas identified 40 genes whose expression levels could separate cases with lymph-node metastasis from those without metastasis. In addition, we compared the expression data with measurements of the sensitivity of surgically dissected NSCLC specimens to six anti-cancer drugs (docetaxel, paclitaxel, irinotecan, cisplatin, gemcitabine, and vinorelbine), as measured by the CD-DST (collagen gel droplet embedded culture-drug sensitivity test) method. We found significant associations between expression levels of dozens of genes and chemosensitivity of NSCLCs. Our results provide valuable information for eventually identifying predictive markers and novel therapeutic target molecules for this type of cancer.

Folate receptor-alpha is a cofactor for cellular entry by Marburg and Ebola viruses

Human infections by Marburg (MBG) and Ebola (EBO) viruses result in lethal hemorrhagic fever. To identify cellular entry factors employed by MBG virus, noninfectible cells transduced with an expression library were challenged with a selectable pseudotype virus packaged by MBG glycoproteins (GP). A cDNA encoding the folate receptor-alpha (FR-alpha) was recovered from cells exhibiting reconstitution of viral entry. A FR-alpha cDNA was recovered in a similar strategy employing EBO pseudotypes. FR-alpha expression in Jurkat cells facilitated MBG or EBO entry, and FR-blocking reagents inhibited infection by MBG or EBO. Finally, FR-alpha bound cells expressing MBG or EBO GP and mediated syncytia formation triggered by MBG GP. Thus, FR-alpha is a significant cofactor for cellular entry for MBG and EBO viruses.

Localized folate and vitamin B-12 deficiency in squamous cell lung cancer is associated with global DNA hypomethylation

We measured the concentrations of folate and vitamin B-12 in paired tissue samples of squamous cell cancer (SCC) and adjacent grossly normal-appearing uninvolved bronchial mucosa (from which SCC developed and also "at risk" of developing SCC) of the lung in 12 subjects to determine the involvement of these vitamins in 1) lung carcinogenesis and 2) global DNA methylation. The folate concentrations were significantly lower in SCCs than in uninvolved tissues (p = 0.03). The vitamin B-12 concentrations were also significantly lower in SCCs than in uninvolved tissues (p = 0.02). The radiolabeled methyl incorporation (inversely related to the degree of in vivo DNA methylation) was significantly higher in SCCs than in uninvolved tissues (p < 0.0001). The correlation between folate and radiolabeled methyl incorporation was inverse and statistically significant in SCCs (p = 0.03). The correlation between vitamin B-12 and radiolabeled methyl incorporation also was inverse and statistically significant in SCCs (p = 0.009). The relationship between tissue vitamin B-12 and DNA methylation was minimal in uninvolved tissues. The relationship between folate and DNA methylation, however, was inverse in uninvolved tissues. In the multiple regression models that included both vitamins, only folate was inversely associated with radiolabeled methyl incorporation in uninvolved and cancerous tissues. These results suggested that folate might be the limiting vitamin for proper DNA methylation in SCC as well as in tissues at risk of developing SCC. Several possible mechanisms of folate deficiency, including inactivation of the vitamin by exposure to carcinogens of cigarette smoke and underexpression or absence of folate receptor in SCCs and associated premalignant lesions, are discussed in light of these findings.

The alpha folate receptor is highly activated in malignant pleural mesothelioma

OBJECTIVE

To determine whether the folate receptor gene is overexpressed in malignant pleural mesothelioma.

METHODS

Differential display analysis was performed with fresh frozen RNA obtained from normal lung, pleura, and mesothelioma. Sixty differentially expressed genes were identified and characterized. One gene that was over-expressed in mesothelioma versus normal tissue was the human alpha folate receptor. In situ hybridization with antisense probes designed on the basis of the sequence of the folate receptor was performed with frozen sections from 61 patients (33 epithelial and 28 mixed or sarcomatoid tumors) with malignant pleural mesothelioma. The controls included normal pleura, normal lung, other cancers, and sense controls for all of the tumors. Northern analysis with a folate receptor probe and immunohistochemical analysis with anti-alpha folate receptor antibodies were also performed.

RESULTS

Forty-four (72%) of the 61 mesothelioma tumors were found to have between 2-fold and 4-fold higher mRNA expression of the folate receptor when compared with the control tissues. The histologic type of the tumor did not affect the rate of folate receptor activation. Northern analysis and immunohistochemical experiments confirmed these findings.

CONCLUSIONS

A majority of mesothelioma tumors examined overexpress the alpha folate receptor protein when compared with normal adjacent tissues. This finding may help explain the observations that antifolate drugs have activity in the treatment of mesothelioma. It also encourages further study of folate receptor-related treatment strategies in this malignancy.

Design of folic acid-conjugated nanoparticles for drug targeting

The new concept developed in this study is the design of poly(ethylene glycol) (PEG)-coated biodegradable nanoparticles coupled to folic acid to target the folate-binding protein; this molecule is the soluble form of the folate receptor that is overexpressed on the surface of many tumoral cells. For this purpose, a novel copolymer, the poly[aminopoly(ethylene glycol)cyanoacrylate-co-hexadecyl cyanoacrylate] [poly(H(2)NPEGCA-co-HDCA)] was synthesized and characterized. Then nanoparticles were prepared by nanoprecipitation of the obtained copolymer, and their size, zeta potential, and surface hydrophobicity were investigated. Nanoparticles were then conjugated to the activated folic acid via PEG terminal amino groups and purified from unreacted products. Finally, the specific interaction between the conjugate folate-nanoparticles and the folate-binding protein was evaluated by surface plasmon resonance. This analysis confirmed a specific binding of the folate-nanoparticles to the folate-binding protein. This interaction did not occur with nonconjugated nanoparticles used as control. Thus, folate-linked nanoparticles represent a potential new drug carrier for tumor cell-selective targeting.

Targeted drug delivery via the folate receptor

The folate receptor is a highly selective tumor marker overexpressed in greater than 90% of ovarian carcinomas. Two general strategies have been developed for the targeted delivery of drugs to folate receptor-positive tumor cells: by coupling to a monoclonal antibody against the receptor and by coupling to a high affinity ligand, folic acid. First, antibodies against the folate receptor, including their fragments and derivatives, have been evaluated for tumor imaging and immunotherapy clinically and have shown significant targeting efficacy in ovarian cancer patients. Folic acid, a high affinity ligand of the folate receptor, retains its receptor binding properties when derivatized via its gamma-carboxyl. Folate conjugation, therefore, presents an alternative method of targeting the folate receptor. This second strategy has been successfully applied in vitro for the receptor-specific delivery of protein toxins, anti-T-cell receptor antibodies, interleukin-2, chemotherapy agents, gamma-emitting radiopharmaceuticals, magnetic resonance imaging contrast agents, liposomal drug carriers, and gene transfer vectors. Low molecular weight radiopharmaceuticals based on folate conjugates showed much more favorable pharmacokinetic properties than radiolabeled antibodies and greater tumor selectivity in folate receptor-positive animal tumor models. The small size, convenient availability, simple conjugation chemistry, and presumed lack of immunogenicity of folic acid make it an ideal ligand for targeted delivery to tumors.

99mTc-ethylenedicysteine-folate: a new tumor imaging agent. Synthesis, labeling and evaluation in animals

It is known that membrane folic acid receptors are responsible for cellular accumulation of folate and folate analogs such as methotrexate and overexpressed on various tumor cells. However, these receptors are highly restricted in normal differentiated tissues. Results of limited in vitro and in vivo animal studies suggest that folate receptors could be a potential target for tumor imaging. This study aimed to develop a 99mTc-labeled folic acid using ethylenedicysteine (EC) as a chelator and evaluate its labeling efficiency and potential use as a tumor seeking agent. Tissue distribution of 99mTc-EC-folate was determined in breast tumor-bearing rats at 20 min, 1, 2, and 4 h (n = 3/time interval, 370 KBq/rat, i.v.). Blocking study was employed to determine receptor-mediated process; 99mTc-EC-folate was co-administrated with 50 and 150 mumol/kg of cold folic acid to tumor-bearing rats. Planar imaging and whole-body autoradiograms were performed. The data was compared to that using 99mTc-EC (control). In animal studies, tumor/blood count density ratios at 20 min-4 h increased from 0.81 +/- 0.09 to 1.23 +/- 0.13 with 99mTc-EC-folate. Conversely, these values showed time-dependent decrease from 0.77 +/- 0.32 to 0.65 +/- 0.01 with 99mTc-EC in the same time period. Tumor/muscle and tumor/blood count density ratios significantly decreased with folic acid co-administrations. Planar images and autoradiograms confirmed that the tumors could be visualized clearly with 99mTc-EC-folate.

Imaging tumor folate receptors using 111In-DTPA-methotrexate

It is known that membrane folic acid receptors are responsible for cellular accumulation of folate and folate analogs, such as methotrexate, and overexpressed on various tumor cells. This study was aimed to develop an 111In labelled DTPA-methotrexate (DTPA-MTX) to image tumor folate receptors in vivo. DTPA-MTX was synthesized by reacting ethylenediamine with MTX. The resulting amino analogue of MTX was reacted with DTPA dianhydride in basic aqueous solution followed by dialysis. Tissue distribution was determined in breast tumor-bearing rats at 0.5, 2, 24, and 48 h (n = 3/time interval). To determine receptor-mediated process 111In-DTPA-MTX was co-administrated with varying blocking doses of cold folate to tumor-bearing rats. Planar imaging and whole-body autoradiograms were performed. The data was compared to that using 111In-DTPA. In animal studies, tumor/blood count density ratios at 0.5-48 h gradually increased from 0.8 +/- 0.32 to 2.2 +/- 0.41 with 111In-DTPA-MTX. Conversely, these values showed time-dependent decrease from 1.19 +/- 0.69 to 0.56 +/- 0.10 with 111In-DTPA in the same time period. Tumor/muscle and tumor/blood count density ratios significantly decreased with high doses of folic acid co-administration. Planar images and autoradiograms confirmed that the tumors could be visualized acceptably with 111In-DTPA-MTX. The results indicate the feasibility of using 111In-DTPA-MTX to image tumors through a folate receptor-mediated process.

Folate-mediated targeting of antineoplastic drugs, imaging agents, and nucleic acids to cancer cells

The receptor for the vitamin, folic acid, is overexpressed on a number of human tumors, including cancers of the ovary, kidney, uterus, testis, brain, colon, lung, and myelocytic blood cells. Conjugates of folic acid linked via its gamma-carboxyl to either a single drug molecule or assembly of molecules can bind to and enter receptor-expressing cancer cells via folate receptor-mediated endocytosis. Because the affinity of folate conjugates for cell surface folate receptors is high (KD approximately 10(-10) M), folic acid derivatization allows the selective delivery of diagnostic and therapeutic agents to cancer cells in the presence of normal cells. This review will summarize studies aimed at folate-mediated targeting of protein toxins, imaging agents, antisense oligodeoxynucleotides, genes, and liposomes specifically to cancer cells in vitro and in vivo.

Folate-maytansinoids: target-selective drugs of low molecular weight

Folate receptor is over-expressed in a variety of carcinomas. To design a cytotoxic drug that would selectively target these carcinomas, we synthesized folate-maytansinoids. These drugs showed high affinity toward folate receptor, appeared to enter cells exclusively via the folate receptor-mediated caveolar pathway and displayed high cytotoxic potency (in the range of 10[-11] to 10[-10] M) and remarkable selectivity for folate receptor-expressing carcinoma cell lines. Folate-maytansinoids represent a new class of tumor-specific agents in which the targeting and the cytotoxic function can be altered independently.

Recognition of normal, neoplastic, and fetal airway epithelial cell membranes by two monoclonal antibodies

The reactivity of two rat monoclonal antibodies was studied. These antibodies, A2R and A2C, bind a 32 kDa alveolar type II cell membrane receptor for surfactant protein A. A2R and A2C also bind apical cell membranes of ciliated and nonciliated cells of the conducting airways. Because this reactivity suggested possible utility in targeting those cells for therapeutic gene transfer, the binding activity of these two antibodies was examined in human tissues. In conducting airways, A2R and A2C bound apical epithelial cell membranes throughout the embryologic period studied: from 15 weeks of gestation, through maturity. Reactivity was more restricted to ciliated cells of the airways as maturation progressed. In the peripheral lung, A2C and A2R only bound most cells in the early developing lung, but mainly type II cells in mature lungs. Other normal tissues recognized by these antibodies included crypt lining cells of the adult and fetal stomach, large bile duct epithelium, and pancreatic acinar cells. All of these cells derive from embryonic foregut endoderm. Other normal tissues, both of endodermal and nonendodermal origin, were negative. Pulmonary carcinomas were studied. A2C and A2R recognized all non-small cell carcinomas of the lung tested. In contrast, none of the small cell carcinomas or carcinoid tumors of the lung were recognized by these antibodies. The function of p32 in these diverse cell types is not clear, but whatever its role in these tissues, antibodies versus p32 may potentially be used to target gene or drug therapy to the normal or malignant cells they recognize.