Antitumor effect of dimethyl itaconate on thymic carcinoma by targeting LDHA-mTOR axis.

AIMS: Thymic carcinoma is a rare type of cancer without an established standard pharmaceutical treatment. This study investigated the antitumor effect of dimethyl itaconate (DI), a cell-permeable derivative of itaconate, on human thymic carcinoma cell line. MAIN METHODS: Human thymic carcinoma cell line Ty82 was used to evaluate the effect of DI on cell viability. Western blotting and immunohistochemistry were performed to determine the molecular mechanism of antitumor effects of DI on Ty82. KEY FINDINGS: DI suppressed cell growth and promoted apoptosis of Ty82. The suppressive effect of DI on Ty82 was mediated by the downregulation of lactate dehydrogenase A (LDHA), and the subsequent decrease in the activity of mechanistic target of rapamycin (mTOR). DI exhibited synergistic antitumor effects with a specific inhibitor of large neutral amino acid transporter 1 (LAT1), an amino acid transporter currently being investigated as a novel target for cancer therapy. SIGNIFICANCE: Our findings demonstrate that DI is a novel potential strategy for thymic carcinoma treatment.

Lenvatinib in patients with advanced or metastatic thymic carcinoma (REMORA): a multicentre, phase 2 trial.

BACKGROUND: Thymic carcinoma is a rare malignant disease and standard treatment for advanced or metastatic thymic carcinoma previously treated with platinum-based chemotherapy has not been established. Lenvatinib is a novel multi-targeted inhibitor of VEGFR, FGFR, RET, c-Kit, and other kinases. The aim of this trial was to assess the activity and safety of lenvatinib as a second-line treatment in thymic carcinoma. METHODS: This single-arm, phase 2 trial done in eight institutions in Japan (five cancer centres, two medical university hospitals, and one public hospital) enrolled patients with pathologically confirmed unresectable advanced or metastatic thymic carcinoma that progressed following at least one platinum-based chemotherapy. Key inclusion criteria were age 20 years or older, at least one measurable lesion as defined by the Response Evaluation Criteria in Solid Tumors version 1.1, and an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients received 24 mg of lenvatinib orally once daily in 4-week cycles until disease progression or occurrence of unacceptable adverse events. The primary endpoint was objective response rate evaluated at the data cutoff date (Feb 22, 2019), by independent central review in the intention-to-treat population. This trial is registered on JMACCT, JMA-IIA00285, and on UMIN-CTR, UMIN000026777. FINDINGS: Between April 21, 2017, and Feb 22, 2018, 42 patients were enrolled and all patients were included in the activity and safety analysis. The median follow-up period was 15·5 months (IQR 13·1-17·5). The objective response rate was 38% (90% CI 25·6-52·0, p<0·0001). 16 (38%) of 42 patients had a partial response and 24 (57%) had stable disease. The most frequent grade 3 treatment-related adverse events were hypertension (27 [64%]) and palmar-plantar erythrodysaesthesia syndrome (three [7%]). No patient died from adverse events. INTERPRETATION: The activity and safety of lenvatinib in patients with advanced or metastatic thymic carcinoma was confirmed. These results suggest that lenvatinib could become a standard treatment option for patients with previously treated advanced or metastatic thymic carcinoma. FUNDING: Center for Clinical Trials, Japan Medical Association.

Expression of cathepsins V and S in thymic epithelial tumors.

Cathepsins are a group of proteolytic enzymes of the endosomal/lysosomal pathway involved in the thymic development of T cells restricted by major histocompatibility complex class II molecules. In the normal thymus, cathepsin V (CTV) and cathepsin S (CTS) are expressed in cortical and medullary epithelial cells, respectively. To investigate whether cathepsins could serve as a diagnostic marker, we performed immunohistochemical analysis for CTV and CTS in 77 cases of thymic epithelial tumors. Almost all cases (59/60) of thymoma expressed CTV, whereas 28 of 60 cases of thymoma expressed CTS. Notably, CTS was expressed in most cases of type A and type AB thymomas, but not in type B thymoma. The expression of cathepsins in type AB thymoma showed a clear correlation with histologic features; CTV was found predominantly in the type B component, and CTS was frequently expressed in the type A component. In thymic carcinoma, CTV was expressed in less than half cases (7/17), and the ratio of CTS-positive cases was equivalent to that of thymoma (8/17). Cases of CTV-negative thymic carcinoma tended to have a higher incidence of recurrence than did CTV-positive cases. Although further studies with a larger number of cases are required to confirm the utility of cathepsin immunostaining, CTV and CTS appear to serve as auxiliary diagnostic and/or prognostic markers in thymic epithelial tumors.

Inhibition of indoleamine 2,3-dioxygenase activity enhances the anti-tumour effects of a Toll-like receptor 7 agonist in an established cancer model.

Toll-like receptor (TLR) agonists have been shown to have anti-tumour activity in basic research and clinical studies. However, TLR agonist monotherapy does not sufficiently eliminate tumours. Activation of the innate immune response by TLR agonists is effective at driving adaptive immunity via interleukin-12 (IL-12) or IL-1, but is counteracted by the simultaneous induction of immunosuppressive cytokines and other molecules, including IL-10, transforming growth factor-ß, and indoleamine 2,3-dioxygenase (IDO). In the present study, we evaluated the anti-cancer effect of the TLR7 agonist, imiquimod (IMQ), in the absence of IDO activity. The administration of IMQ in IDO knockout (KO) mice inoculated with tumour cells significantly suppressed tumour progression compared with that in wild-type (WT) mice, and improved the survival rate. Moreover, injection with IMQ enhanced the tumour antigen-specific T helper type 1 response in IDO-KO mice with tumours. Combination therapy with IMQ and an IDO inhibitor also significantly inhibited tumour growth. Our results indicated that the enhancement of IDO expression with TLR agonists in cancer treatment might impair host anti-tumour immunity while the inhibition of IDO could enhance the therapeutic efficacy of TLR agonists via the increase of T helper type 1 immune response.

Expression and clinical significance of protein tyrosine phosphatase nonreceptor 22 in resected thymoma.

BACKGROUND: To investigate the expression and clinical significance of protein tyrosine phosphatase nonreceptor 22 (PTPN22) in thymoma, as well as the relationship between thymoma and myasthenia gravis (MG). METHODS: The expression of PTPN22 in normal thymus (35 cases), thymoma without MG (50 cases), and thymoma with MG (45 cases) treated with surgery were detected by the EnVision two-step immunohistochemical staining method. We analyzed the relationship of thymoma with MG as well as some clinical factors, such as the Osserman classification for MC, age, gender, and course of disease before surgery. All data were analyzed using the SPSS software. RESULTS: The positive rates of PTPN22 expression in thymoma with and without MG were 11.1% (5/45) and 24.0% (12/50), respectively, which were significantly lower than in the normal thymus (74.3%, 26/35). A significant difference was found between the positive rates of thymoma with and without MG. The expression level of PTPN22 had no relation to thymoma with MG and to the Osserman classification for MG, age, gender, and course of disease of the patient. CONCLUSIONS: Minimal or no PTPN22 expression in thymoma may play an important role in the development of thymoma with MG, provide basis for further genetic research.

Thymoma patients treated in a phase I clinic at MD Anderson Cancer Center: responses to mTOR inhibitors and molecular analyses.

BACKGROUND: Thymomas and thymic carcinoma are rare tumors with no approved therapies. Our purpose was to analyze the molecular features and outcomes of patients referred to the Clinical Center for Targeted Therapy (Phase I Clinic). METHODS: We retrospectively reviewed the medical records of consecutive referred patients with advanced/metastatic thymoma or thymic carcinoma RESULTS: Twenty-one patients were identified (median age 52 years; 10 women; median number of prior systemic therapies = 2). Six of 10 patients (60%) treated with mTOR inhibitor combination regimens achieved stable disease (SD) ≥12 months or a partial response (PR). For patients treated on mTOR inhibitor regimens (N = 10), median time to treatment failure (TTF) was 11.6 months versus 2.3 months on last conventional regimen prior to referral (p=0.024). Molecular analyses (performed by next generation sequencing in seven patients and single polymerase chain reaction (PCR)-based assays in an additional six patients) showed diverse actionable mutations: PIK3CA (1 of 12 tested; 8%); EGFR (1 of 13; 8%); RET (1 of 7; 14%); and AKT1 (1 of 7; 14%). Of two patients with PIK3CA or AKT1 mutations, one was treated with an mTOR inhibitor-based regimen and achieved 26% regression with a TTF of 17 months. CONCLUSION: Patients with advanced/metastatic thymoma or thymic carcinoma demonstrated prolonged TTF on mTOR inhibitor-based therapy as compared to prior conventional treatment. Heterogeneity in actionable molecular aberrations was observed, suggesting that multi-assay molecular profiling and individualizing treatment merits investigation.

Messenger RNA and protein expression of thymidylate synthase and DNA repair genes in thymic tumors.

BACKGROUND: Thymic epithelial tumors include several entities with different biologic behavior. Chemotherapy is indicated in advanced disease, but limited data exist on gene expression correlation with the response to chemotherapeutic agents. PATIENTS AND METHODS: A series of 69 thymic neoplasms (7 A-, 6 AB-, 6 B1-, 10 B2-, 14 B3-thymomas, 22 carcinomas and 4 combined tumors) was collected to assess gene expression of thymidylate synthase (TS), excision repair cross complementing-1 (ERCC1), ribonucleotide reductase subunit 1 (RRM1), topoisomerase 2α (TOP2A) and mTOR. RESULTS: A strong linear correlation between TS gene and protein expression was observed (P<0.0001, R=0.40). TS expression was significantly lower in pure A-thymomas and thymic carcinomas (P<0.0001) and progressively decreasing from B1-type to thymic carcinomas (B1>B2>B3>C; P<0.0001). RRM1 and TOP2A mRNA expression levels were significantly correlated with TS levels (both P=0.03) with a similar trend of expression among histotypes. RRM1 and TOP2A high levels were significantly correlated with high TS (P=0.03) and low tumor stages (I-II) (P<0.0001 and P<0.01, respectively). No relevant changes of ERCC1 and mTOR were detected. CONCLUSIONS: Low TS and, to a minor extent, RRM1 and TOP2A expression were detected in aggressive thymic tumors. These findings should be prospectively considered in selecting the most appropriate chemotherapy.

Expression of proteasome subunit ß5t in thymic epithelial tumors.

Recently, a proteasome ß subunit expressed exclusively in thymic cortical epithelial cells was discovered in mice and humans. This subunit, designated ß5t, is a component of the thymoproteasome, a specialized type of proteasome implicated in thymic positive selection. To investigate whether ß5t could serve as a marker for the differential diagnosis of thymic epithelial tumors, we performed immunohistochemical analysis using anti-ß5t antibody in 54 cases of thymic epithelial tumors comprising 41 cases of thymomas and 13 cases of thymic carcinomas. ß5t was detected in the neoplastic epithelial cells of thymomas. Among the subtypes of thymoma, expression of ß5t was observed in most cases of type B thymoma (20 of 21) but not in type A thymomas (0 of 3). In type AB thymomas, ß5t expression was variable (6 of 17). Type B3 thymomas (4 cases) were positive for ß5t but negative for CD5, c-kit, and glucose transporter 1 (GLUT-1), which are known as diagnostic markers for thymic carcinomas. In contrast, thymic carcinomas were negative for ß5t (0 of 13) but expressed at least one and usually all of CD5, c-kit, and GLUT-1. Thus, ß5t and CD5/c-kit/GLUT-1 were differentially expressed in type B3 thymoma and thymic carcinoma. We tested ß5t expression in 39 cases of tumors arising from other organs, which showed the specific expression of ß5t in thymic epithelial tumors. This study demonstrates that ß5t is expressed in most type B and in some type AB thymomas and is a marker useful in differentiating type B3 thymomas from thymic carcinomas when used in combination with other diagnostic markers.

Sunitinib in metastatic thymic carcinomas: laboratory findings and initial clinical experience.

BACKGROUND: Thymic carcinoma (TC) is a rare aggressive tumour. Median survival with current treatments is only 2 years. Sunitinib is a multi-targeted tyrosine kinase inhibitor that has shown benefit in various other cancers. METHODS: Laboratory analyses of snap-frozen tumour tissues were performed to detect activation and genetic mutations of receptor tyrosine kinases (RTKs) in TC samples. On the basis of molecular analyses showing activation of multiple RTKs in their tumour, four patients with metastatic TCs refractory to conventional therapies were treated with sunitinib according to standard protocols. RESULTS: RTK analysis in three of the patients showed activation of multiple RTKs, including platelet-derived growth factor-beta and vascular endothelial growth factor 3. Mutations of EGFR, c-KIT, KRAS, and BRAF genes were not found. Administration of sunitinib yielded a partial remission (lasting 2 to 18+ months) according to the RECIST criteria in three patients and stable disease with excellent metabolic response in 18F-FDG-PET in another one. The overall survival with sunitinib treatment ranges from 4 to 40+ months. Withdrawal of the drug in one patient prompted rapid tumour progression that could be controlled by re-administration of sunitinib. CONCLUSIONS: Sunitinib is an active treatment for metastatic TC. A panel of molecular analyses may be warranted for optimal patient selection.

Anti-CD44 induces apoptosis in T lymphoma via mitochondrial depolarization.

A blockade of CD44 can interfere with haematopoietic and leukemic stem cell homing, the latter being considered as a therapeutic option in haematological malignancies. We here aimed to explore the molecular mechanism underlying the therapeutic efficacy of anti-CD44. We noted that in irradiated mice reconstituted with a bone marrow cell transplant, anti-CD44 exerts a stronger effect on haematopoietic reconstitution than on T lymphoma (EL4) growth. Nonetheless, in the non-reconstituted mouse anti-CD44 suffices for a prolonged survival of EL4-bearing mice, where anti-CD44-prohibited homing actively drives EL4 cells into apoptosis. In vitro, a CD44 occupancy results in a 2-4-fold increase in apoptotic EL4 cells. Death receptor expression (CD95, TRAIL, TNFRI) remains unaltered and CD95 cross-linking-mediated apoptosis is not affected. Instead, CD44 ligation promotes mitochondrial depolarization that is accompanied by caspase-9 cleavage and is inhibited in the presence of a caspase-9 inhibitor. Apoptosis becomes initiated by activation of CD44-associated phosphatase 2A (PP2A) and proceeds via ERK1/2 dephosphorylation without ERK1/2 degradation. Accordingly, CD44-induced apoptosis could be mimicked by ERK1/2 inhibition, that also promotes EL4 cell apoptosis through the mitochondrial pathway. Thus, during haematopoietic stem cell reconstitution care should be taken not to interfere by a blockade of CD44 with haematopoiesis, which could be circumvented by selectively targeting leukemic CD44 isoforms. Beyond homing/settlement in the bone marrow niche, anti-CD44 drives leukemic T cells into apoptosis via the mitochondrial death pathway by CD44 associating with PP2A. Uncovering this new pathway of CD44-induced leukemic cell death provides new options of therapeutic interference.

Identification of an indispensable role for tyrosine kinase 2 in CTL-mediated tumor surveillance.

We showed previously that Tyk2(-/-) natural killer cells lack the ability to lyse leukemic cells. As a consequence, the animals are leukemia prone. Here, we show that the impaired tumor surveillance extends to T cells. Challenging Tyk2(-/-) mice with EL4 thymoma significantly decreased disease latency. The crucial role of Tyk2 for CTL function was further characterized using the ovalbumin-expressing EG7 cells. Tyk2(-/-) OT-1 mice developed EG7-induced tumors significantly faster compared with wild-type (wt) controls. In vivo assays confirmed the defect in CD8(+) cytotoxicity on Tyk2 deficiency and clearly linked it to type I IFN signaling. An impaired CTL activity was only observed in IFNAR1(-/-) animals but not on IFNgamma or IL12p35 deficiency. Accordingly, EG7-induced tumors grew faster in IFNAR1(-/-) and Tyk2(-/-) but not in IFNgamma(-/-) or IL12p35(-/-) mice. Adoptive transfer experiments defined a key role of Tyk2 in CTL-mediated tumor surveillance. In contrast to wt OT-1 cells, Tyk2(-/-) OT-1 T cells were incapable of controlling EG7-induced tumor growth.

COX-2 upregulation in thymomas and thymic carcinomas.

The treatment of advanced stage thymomas and thymic carcinomas is a multimodal therapy. New therapeutic targets are currently under investigation, including the epidermal growth factor receptor (EGFR) as well as KIT. A number of studies have shown protumorigenic potential of Cyclooxygenase-2 (COX-2) in a variety of human malignancies, but so far it is unknown whether COX-2 is expressed in primary malignancies of the thymus. Using tissue microarrays, the expression of COX-2, microsomal-PGES-1 and -PGES-2 (mPGES-1 and mPGES-2), as well as EGFR was evaluated in different subtypes of thymoma and thymic carcinomas. COX-2 was expressed in all subtypes as determined by immunohistochemistry. Some cases of type B2 and thymic carcinomas had COX-2 staining levels classified as mild to moderate. However, when measuring the optical color intensity, no significant differences could be detected. Concerning the expression levels, a weak correlation between the expression of COX-2, mPGES-1 and mPGES-2 as well as EGFR was found. Furthermore, additional cases of thymomas and thymic carcinomas were analyzed by COX-2 Western immunoblot analysis and were compared to normal thymi. The analysis showed that thymomas and thymic carcinomas had a significantly stronger COX-2 expression than that of the normal thymi (p < 0.04). In summary, COX-2 is expressed in all subtypes of thymomas and thymic carcinomas and thus represents, in addition to EGFR and KIT, a potential therapeutic target. Further studies are needed in order to determine whether a combined therapy using COX-2 inhibitors in addition to the evolving anti-EGFR antibody therapy may be considered as a treatment option.

[Correlation between MMP-2 activation and MT1-MMP mRNA expression in thymic epithelial tumors].

OBJECTIVE: To study the relationship between activation of pro-MMP-2 and expression of matrix metalloproteinases (MMP)-2, MT1-MMP and tissue inhibitor of metalloproteinases (TIMP)-2 mRNA in thymoma and thymic carcinoma; and to study the molecular mechanism of invasion and metastasis of thymic epithelial tumors. METHODS: Fresh tissue specimens of thymoma, thymic carcinoma and normal thymus were included. The mRNA expression of MMP-2, MT1-MMP and TIMP-2 were analyzed by real-time reverse transcription polymerase chain reaction. The pro-MMP-2 activation ratio and its localization were determined by gelatin zymography and film in-situ gelatin-Zymography, respectively. Correlation of mRNA expression of MMP-2, MT1-MMP and TIMP-2 was investigated in tumors with different histological subtypes and clinical stages. RESULTS: There were no significant differences in the expressions of MMP-2, MT1-MMP and TIMP-2 mRNA between I and II stage or III and IV stage thymomas (P > 0.05). However, significant differences of the expressions were observed between three tumor groups: I-II stage, III-IV stage and thymic carcinomas (P < 0.005), and between three histological subtypes: AB-B1 (lymphocyte-rich and mixed types), B2-B3 (cortical and predominantly polygonal cells types) and thymic carcinomas (P < 0.05). Expression levels of MT1-MMP and TIMP-2 mRNA were correlated with pro-MMP-2 activation ratio (Spearman rank correlation: r = 0.7235, r = 0.7647, P < 0.005). The expression of MMP-9 did not show significant differences between thymomas and thymic carcinomas. CONCLUSIONS: MMP-2, MT1-MMP and TIMP-2 mRNA expression levels are correlated with the histologic subtypes and clinical stages of thymoma. The mRNA expressions of MT1-MMP and TIMP-2 are correlated with the activation ratio of pro-MMP-2. It is speculated that upregulation of MT1-MMP gene expression may induce an activation of pro-MMP-2 through TIMP-2.

Tumor cell-derived nitric oxide is involved in the immune-rejection of an immunogenic murine lymphoma.

The roles played by host-derived nitric oxide (NO) in the growth and subsequent immune rejection of a immunogenic murine lymphoma were investigated by growing the tumor in mice in which the gene for either inducible NO synthase (iNOS) or endothelial NOS (eNOS) had been ablated. This showed that NO from tumor-infiltrating host cells had no significant effect on either tumor growth or immune rejection, although measurements of tumor nitrite levels and protein nitration showed that there had been significant NO production in the rejected tumors, in both the eNOS and iNOS knockout mice. Inhibition of both tumor and host NOS activities, with an iNOS-selective inhibitor (1400W), a nonselective NOS inhibitor [Nomega-nitro-L-arginine methyl ester (L-NAME)], or scavenging NO with a ruthenium-based scavenger, significantly delayed tumor rejection, while having no appreciable effect on tumor growth. Incubation of tumor cells with medium taken from cultured splenocytes, that had been isolated from immunized animals and activated by incubating them with irradiated tumor cells, resulted in an increase in tumor cell NOS activity and an increase in tumor cell apoptosis, which could be inhibited using L-NAME. We propose that, during the immune rejection of this tumor model, there is induction of tumor NOS activity by cytokines secreted by activated lymphocytes within the tumor and that this results in increased levels of tumor NO that induce tumor cell apoptosis and facilitate immune rejection of the tumor.

Increased expression of mitochondrial peroxiredoxin-3 (thioredoxin peroxidase-2) protects cancer cells against hypoxia and drug-induced hydrogen peroxide-dependent apoptosis.

Peroxiredoxin-3 (Prdx3) is a mitochondrial member of the antioxidant family of thioredoxin peroxidases that uses mitochondrial thioredoxin-2 (Trx2) as a source of reducing equivalents to scavenge hydrogen peroxide (H(2)O(2)). Low levels of H(2)O(2) produced by the mitochondria regulate physiological processes, including cell proliferation, while high levels of H(2)O(2) are toxic to the cell and cause apoptosis. WEHI7.2 thymoma cells with stable overexpression of Prdx3 displayed decreased levels of cellular H(2)O(2) and decreased cell proliferation without a change in basal levels of apoptosis. Prdx3-transfected cells showed a marked resistance to hypoxia-induced H(2)O(2) formation and apoptosis. Prdx3 overexpression also protected the cells against apoptosis caused by H(2)O(2), t-butylhydroperoxide, and the anticancer drug imexon, but not by dexamethasone. Thus, mitochondrial Prdx3 is an important cellular antioxidant that regulates physiological levels of H(2)O(2), leading to decreased cell growth while protecting cells from the apoptosis-inducing effects of high levels of H(2)O(2).

Thymidylate synthase and dihydropyrimidine dehydrogenase mRNA levels in thymoma.

PURPOSE: Thymoma is one of the most common solid tumors in the mediastinum. However, there is no definitive consensus regarding the optimal adjuvant chemotherapy for advanced thymoma. METHODS: To predict tumor sensitivity to 5-fluorouracil (5-FU) in thymoma, we investigated the mRNA levels of thymidylate synthase (TS), the key enzyme that catalyzes the methylation of deoxyuridine monophosphate, and correlates with the resistance of 5-FU and dihydropyrimidine dehydrogenase (DPD), which degrades 5-FU in thymoma. We used real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) using the LightCycler to monitor the TS and DPD gene expression levels in thymoma tissue specimens from patients, coamplified with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as an internal standard. RESULTS: In the resected tumor specimens, TS and DPD mRNA levels were 3.876 and 14.651, respectively. Both the TS and DPD mRNA levels were significantly higher in the tumor tissue specimens than in the normal adjacent thymus tissue specimens. No significant correlations were observed between the TS or DPD levels and other clinicopathological factors. CONCLUSIONS: The combined use of measurements of TS and DPD mRNA levels using real-time RT-PCR analyses may provide an indication of the selective cytoxicity of 5-FU on thymoma. In general, 5-FU itself is not considered to be a useful treatment for thymoma. The usufulness of DPD-inhibiting fluoropyrimidine (DIF) drugs for thymoma should therefore be further considered.

Immunobiochemical characteristics of IgG antibodies in myasthenia.

The abzyme activity of mAb35 and pQ1-209 was compared with that of normal rabbit IgG and serum IgG from patients with various forms of myasthenia. It was found that mAb35 and pQ1-209 and IgG from patients with myasthenia possess catalytic activity. IgG from myasthenia patients with thymomas possess creatine phosphokinase activity, which 2-fold surpassed the control.

[Expression of telomerase activity in thymoma and thymiccarcinoma tissues; a clinicopathological study].

BACKGROUND: Telomerase is a nucleoprotein complex that caps the physical termini of all eukaryotic chromosomes. It is suggested that telomerase play important roles in unlimited cell division acquisition of the malignant phenotype. We studied the relation of telomerase activity in thymoma and thymic carcinoma to the clinicopathological features of these lesions. METHODS: Tissue specimens were surgically resected from patients with thymoma and thymic carcinoma. Telomerase activity was evaluated according to a modified telomeric repeat amplification protocol (TRAP) assay. RESULTS: Telomerase activity was detected in all thymic epithelial tumors. The activity (mean +/- SD: unit/microgram.protein) in thymoma (n = 17) was significantly higher than that in thymic carcinoma (n = 7) [431.8 +/- 400.1 vs. 68.8 +/- 39.8: p < 0.01]. Telomerase activities in thymoma and thymic carcinoma were significantly higher than that in primary lung adenocarcinoma (33.5 +/- 39.2: n = 47), studied as control (p < 0.01). In patients with thymoma, telomerase activity did not correlate with tumor stage according to Masaoka classification (p = 0.776). In patients with thymic carcinoma, however, telomerase activity positively correlated with tumor stage (p = 0.02). In thymoma, telomerase activity positively correlated with the ratio of induced lymphocytes according to Rosai's classification (p = 0.045). CONCLUSION: In thymoma, telomerase activity reflects the presence of immature T-cell lymphocytes in tumor tissue rather than tumor stage or malignant phenotype. In thymic carcinoma, telomerase activity derived directly from cancer cells may relate to tumor stage.