Hans-Georg Kopp

Tyrosine Kinase Inhibitors - A Review on Pharmacology, Metabolism and Side Effects

Tyrosine kinase inhibitors (TKI) are effective in the targeted treatment of various malignancies. Imatinib was the first to be introduced into clinical oncology, and it was followed by drugs such as gefitinib, erlotinib, sorafenib, sunitinib, and dasatinib. Although they share the same mechanism of action, namely competitive ATP inhibition at the catalytic binding site of tyrosine kinase, they differ from each other in the spectrum of targeted kinases, their pharmacokinetics as well as substance-specific adverse effects. With variations from drug to drug, tyrosine kinase inhibitors cause skin toxicity, including folliculitis, in more than 50% of patients. Among the tyrosine kinase inhibitors that are commercially available as yet, the agents that target EGFR, erlotinib and gefitinib, display the broadest spectrum of adverse effects on skin and hair, including folliculitis, paronychia, facial hair growth, facial erythema, and varying forms of frontal alopecia. In contrast, folliculitis is not common during administration of sorafenib and sunitinib, which target VEGFR, PDGFR, FLT3, and others, whereas both agents have been associated with subungual splinter hemorrhages. Periorbital edema is a common adverse effect of imatinib. Besides the haematological side effects of most of TKIs like anemia, thrombopenia and neutropenia, the most common extra-heamatologic adverse effects are edema, nausea, hypothyroidism, vomiting and diarrhea. Regarding possible long term effects, recently cardiac toxicity with congestive heart failure is under debate in patients receiving imatinib and sunitinib therapy; however, this observation was probably relate to patients selection, although, TKIs overall appear to be a very well tolerated drug class.

Contribution of endothelial progenitors and proangiogenic hematopoietic cells to vascularization of tumor and ischemic tissue.

Purpose of reviewDuring the last several years, a substantial amount of evidence from animal as well as human studies has advanced our knowledge of how bone marrow derived cells contribute to neoangiogenesis. In the light of recent findings, we may have to redefine our thinking of endothelial cells as well as of perivascular mural cells. Recent findingsInflammatory hematopoietic cells, such as macrophages, have been shown to promote neoangiogenesis during tumor growth and wound healing. Dendritic cells, B lymphocytes, monocytes, and other immune cells have also been found to be recruited to neoangiogenic niches and to support neovessel formation. These findings have led to the concept that subsets of hematopoietic cells comprise proangiogenic cells that drive adult revascularization processes. While evidence of the importance of endothelial progenitor cells in adult vasculogenesis increased further, the role of these comobilized hematopoietic cells has been intensely studied in the last few years. SummaryAngiogenic factors promote mobilization of vascular endothelial growth factor receptor 1-positive hematopoietic cells through matrix metalloproteinase-9 mediated release of soluble kit-ligand and recruit these proangiogenic cells to areas of hypoxia, where perivascular mural cells present stromal-derived factor 1 (CXCL-12) as an important retention signal. The same factors are possibly involved in mobilization of vascular endothelial growth factor receptor 2-positive endothelial precursors that may participate in neovessel formation. The complete characterization of mechanisms, mediators and signaling pathways involved in these processes will provide novel targets for both anti and proangiogenic therapeutic strategies.

Platelet-Derived Transforming Growth Factor-β Down-Regulates NKG2D Thereby Inhibiting Natural Killer Cell Antitumor Reactivity

Natural killer (NK) cells play an important role in cancer immunosurveillance and may prevent tumor progression and metastasis due to their ability to mediate direct cellular cytotoxicity and by releasing immunoregulatory cytokines, which shape adaptive immune responses. Their reactivity is governed by various activating and inhibitory molecules expressed on target cells and reciprocal interactions with other hematopoietic cells such as dendritic cells. In mice, thrombocytopenia inhibits metastasis, and this is reversed by NK cell depletion, suggesting that platelets are an important additional player in NK cell-tumor interaction. Moreover, it has been shown that metastasizing tumor cells do not travel through the blood alone but are rapidly coated by platelets. However, the knowledge about the molecular mechanisms by which platelets influence NK cells is fragmentary at best. Here we show that platelet-derived soluble factors, secreted on coating of tumor cells or after stimulation with classic platelet agonists, impair NK cell antitumor reactivity resulting in diminished granule mobilization, cytotoxicity, and IFN-gamma production. The impaired NK cell reactivity was not due to induction of apoptosis but mediated by down-regulation of the activating immunoreceptor natural killer group 2, member D (NKG2D) on NK cells by platelet-derived transforming growth factor beta (TGF-beta). Neutralization of TGF-beta in platelet releasate not only prevented NKG2D down-regulation but also restored NK cell antitumor reactivity. Thus, our data elucidate the molecular basis of the previously described influence of platelets on NK cell antitumor reactivity and suggest that therapeutic intervention in tumor cell-platelet interaction and the resulting TGF-beta release by platelets may serve to enhance antitumor immunity.

Thrombospondins deployed by thrombopoietic cells determine angiogenic switch and extent of revascularization

Thrombopoietic cells may differentially promote or inhibit tissue vascularization by releasing both pro- and antiangiogenic factors. However, the molecular determinants controlling the angiogenic phenotype of thrombopoietic cells remain unknown. Here, we show that expression and release of thrombospondins (TSPs) by megakaryocytes and platelets function as a major antiangiogenic switch. TSPs inhibited thrombopoiesis, diminished bone marrow microvascular reconstruction following myelosuppression, and limited the extent of revascularization in a model of hind limb ischemia. We demonstrate that thrombopoietic recovery following myelosuppression was significantly enhanced in mice deficient in both TSP1 and TSP2 (TSP-DKO mice) in comparison with WT mice. Megakaryocyte and platelet levels in TSP-DKO mice were rapidly restored, thereby accelerating revascularization of myelosuppressed bone marrow and ischemic hind limbs. In addition, thrombopoietic cells derived from TSP-DKO mice were more effective in supporting neoangiogenesis in Matrigel plugs. The proangiogenic activity of TSP-DKO thrombopoietic cells was mediated through activation of MMP-9 and enhanced release of stromal cell-derived factor 1. Thus, TSP-deficient thrombopoietic cells function as proangiogenic agents, accelerating hemangiogenesis within the marrow and revascularization of ischemic hind limbs. As such, interference with the release of cellular stores of TSPs may be clinically effective in augmenting neoangiogenesis.

Steady-state neutrophil homeostasis is dependent on TLR4/TRIF signaling

UNLABELLED Polymorphonuclear neutrophil granulocytes (neutrophils) are tightly controlled by an incompletely understood homeostatic feedback loop adjusting the marrows supply to peripheral needs. Although it has long been known that marrow cellularity is inversely correlated with G-CSF levels, the mechanism linking peripheral clearance to production remains unknown. Herein, the feedback response to antibody induced neutropenia is characterized to consist of G-CSF–dependent shifts of marrow hematopoietic progenitor populations including expansion of the lin-/Sca-1/c-kit (LSK) and granulocyte macrophage progenitor (GMP) compartments at the expense of thrombopoietic and red cell precursors. Evidence is provided that positive feedback regulation is independent from commensal germs as well as T, B, and NK cells. However, in vivo feedback is impaired in TLR4-/- and TRIF-/-, but not MyD88-/- animals. In conclusion, steady-state neutrophil homeostasis is G-CSF–dependent and regulated through pattern-recognition receptors,thereby directly linking TLR-triggering to granulopoiesis. KEY POINTS Steady-state and emergency granulopoiesis are both dependent on TLR signaling.

Glucocorticoid-induced TNFR-related (GITR) protein and its ligand in antitumor immunity: functional role and therapeutic modulation.

The ability of the tumor necrosis factor receptor (TNFR) family member GITR to modulate immune responses has been the subject of multiple studies. Initially thought to be critically involved in governing functions of regulatory T cells, GITR and its ligand GITRL have meanwhile been found to modulate the reactivity of various different cell types and to influence a broad variety of immunological conditions including the immune response against tumors. Not only GITR, but also GITRL is capable of transducing signals, and the consequences of GITR-GITRL interaction may vary among different effector cell types, differ upon signal transduction via the receptor, the ligand, or both, depend on the level of an ongoing immune response, and even differ among mice and men. In this paper, we address available data on GITR and its ligand in immune responses and discuss the role and potential therapeutic modulation of this molecule system in antitumor immunity.

Functional heterogeneity of the bone marrow vascular niche.

Sinusoidal endothelial cells (SECs) comprise the platform where trafficking into and out of the BM occurs and where hematopoietic stem and progenitor cells (HSPC) harbor and receive cues for self‐renewal, survival, and differentiation. Therefore, SECs are referred to as a bone marrow vascular niche (BMVN). Hematopoietic regeneration has been shown to occur only with concurrent angiogenic regeneration. However, there are still not sufficient means to identify and isolate SECs, therefore the “niche endothelial cell” remains incompletely characterized. VEGF‐receptor‐3 (VEGFR3) is expressed exclusively by the SECs, while Sca1 and Tie2 are only expressed on the VEGFR3− arteriolar endothelium. We previously demonstrated the importance of vascular recovery in hematopoietic regeneration from myelosuppression due to cytotoxic agents or whole‐body irradiation. Therefore to establish the functional importance of SECs, the mechanisms underlying BMVN regeneration were examined utilizing a 5‐fluorouracil (5‐FU) myelosuppression model of vascular damage. Injection of antibodies against murine VEGFR‐1 and ‐2 had no significant effect on hemangiogenic recovery. However, when soluble VEGFR‐1, a decoy receptor for VEGF‐A and PlGF, was injected after 5‐FU, both angiogenic remodeling and regeneration of megakaryopoiesis were delayed. In conclusion, we show that the bone marrow vasculature comprises heterogeneous compartments. SECs are distinguished from arterioles by unique immunophenotypes. Regeneration of damaged SECs is the rate‐limiting step in hematopoietic regeneration from myelosuppressive therapy. Novel, high‐efficiency VEGF‐binding drugs in combination with chemotherapeutic agents may lead to cases of prolonged cytopenia.

Differential changes in platelet VEGF, Tsp, CXCL12, and CXCL4 in patients with metastatic cancer

Data from animal studies indicate that platelets play a key role in tumor dissemination and metastasis. We therefore hypothesized that metastastic cancer patients may display a specific platelet phenotype. Percentage of activated, p-selectin positive platelets as well as platelet contents (i.e., plasma and platelet count-corrected serum levels of VEGF-A, CXCL12, CXCL4, and thrombospondin-1) were analyzed in 43 patients with newly diagnosed metastatic disease prior to treatment. Tumor patients had increased platelet counts and significantly elevated percentages of activated platelets. Moreover, the platelet content of VEGF-A in cancer patients was significantly increased compared to healthy controls, while thrombospondin-1, CXCL12 and CXCL4 were significantly decreased. Our data contain several unexpected results: firstly, CXCL12 was found in minute quantities in the serum as compared with murine studies. Secondly, CXCL4, which was found by mass spectrometry to be the single massively upregulated intraplatelet chemokine in mice after tumor xenotransplantation, was decreased in tumor patient platelets. While increased contents of VEGF-A have been attributed to platelet scavenger activity, the differential decrease of specific platelet contents may be due to differential secretion or altered megakaryopoiesis in metastatic cancer patients.

Current insights into neutrophil homeostasis

Neutrophil granulocytes represent the first immunologic barrier against invading pathogens, and neutropenia predisposes to infection. However, neutrophils may also cause significant collateral inflammatory damage. Therefore, neutrophil numbers are tightly regulated by an incompletely understood homeostatic feedback loop adjusting the marrows supply to peripheral needs. Granulocyte colony‐stimulating factor (G‐CSF) is accepted to be the major determinant of neutrophil production, and G‐CSF levels have, soon after its discovery, been described to be inversely correlated with neutrophil counts. A neutrophil sensor, or “neutrostat,” has, therefore, been postulated. The prevailing feedback hypothesis was established in adhesion molecule–deficient mice; it includes macrophages and Th17 cells, which determine G‐CSF levels in response to the number of peripherally transmigrated, apoptosing neutrophils. Recent work has deepened our understanding of homeostatic regulation of neutrophil granulopoiesis, but there are still inconsistent findings and unresolved questions when it comes to a plausible hypothesis, similar to the feedback control models of red cell or platelet homeostasis.

Advances in the Treatment of Testicular Cancer

Testicular cancer is the most common solid tumour in young men, and the treatment of testicular germ cell tumours (TGCT) has been called a success story of medical oncology, germ cell cancer being regarded as the “model of a curable neoplasm”. Even with metastatic disease, high cure rates can be achieved: the overall 5-year survival for all stages of TGCT is approximately 80%. Today, elaborate systems for prognostic evaluation for gonadal and extragonadal germ cell tumours facilitate the choice of the most appropriate therapy for individual patients. In doing so, the ultimate goal of treatment is tumour-free survival for any patient with TGCT.This goal has already been reached for >99% of the patients with early-stage tumours, as well as for the majority of patients with advanced disease (56% of patients with metastases are considered to have a good prognosis at the time of diagnosis; the 5-year survival rate for this group is 90%). However, patients with ‘intermediate’ or ‘poor’ prognosis at the time of diagnosis, as well as patients with relapsed disease after cisplatin-containing therapy, still have an unsatisfactorily low 5-year survival rate after standard therapy with PEB (cisplatin, etoposide, bleomycin) of only 80%, 45–55% and 20–25%, respectively.Therefore, our goals must be (i) to limit acute and chronic toxicity by avoiding overtreatment for patients with localised disease and/or good prognosis with advanced disease; and (ii) to identify patients with poor prognosis and treat them in specialised centres, where not only is optimal interdisciplinary care available but new treatment strategies are being applied. For example, tandem high-dosechemotherapy regimens might be effective in achieving higher cure rates in these patients.