Tasonermin

In vivo antitumour activity is probably based on direct and indirect effects.

Direct inhibition of tumour cell proliferation

Tasonermin is cytotoxic or cytostatic in vitro for a variety of tumour cell lines of different histogenesis.

Direct effects on tumour vasculature

Tasonermin affects the morphology and reduces proliferation of endothelial cells and modifies expression of specific cell surface and secretory proteins (including adhesion molecules and proteins modulating coagulation, interleukins and haematopoietic growth factors). These changes in turn lead to a procoagulant state, resulting in microvascular thrombosis. Further, adherence and extravasation of leukocytes is increased, leading to infiltration of the tumour by lymphocytes, monocytes, and granulocytes. The reason for the differential sensitivity of the tumour vasculature (high) versus normal vasculature (low) are currently unknown.

Indirect and direct immunomodulation

Tasonermin has profound effects on cellular components of the immune system. Proliferation of activated B- and T-lymphocytes, development of cytotoxic T-cells and immunoglobulin-secreting cells is enhanced, monocytes/macrophages are activated for killing of tumour cells, granulocytes are activated to display enhanced phagocytic activity, respiratory burst and degranulation, and adherence to endothelium. Further, in addition to its direct effects, tasonermin modulates immune responses by inducing production of cytokines as well as low molecular weight mediators (prostaglandins, platelet activating factor). Several lines of evidence suggest that these immunomodulatory activities are of relevance for the antitumour effects; e.g. the antitumour activities of tasonermin are much less pronounced in immunodeficient animals. Further, animals that reject experimental tumours following tasonermin treatment may develop specific immunity for this tumour cell type.

Pharmacodynamic effects

Tasonermin has been shown to be active in the classic assay for tumour necrosis factor, producing haemorrhagic necrosis of tumour nodules in murine syngeneic and human xenogeneic tumour systems after local or systemic injection. The systemic application of tasonermin is limited by its toxic effects, the effective dose predicted from preclinical studies being substantially higher than the observed human maximum tolerated dose.

Systemic pharmacokinetics

The systemic pharmacokinetic information on tasonermin is sparse. A dose-dependency has been observed as indicated by a decrease in clearance and an increase in half-life at increasing doses. The terminal half-life at the maximum tolerated intravenous dose (150 µg/m²) was 15-30 min.

Pharmacokinetics in ILP

ILP allows the administration of high and fairly stable concentrations of tasonermin to the limb. Data obtained from 51 ILP patients demonstrated maximum concentrations of tasonermin in the perfusion circuit are reached 30 min after the onset of ILP and range between 3000 and 4000 ng/ml. Under conditions of less than 2% systemic leakage (observed in 38 of 51 patients), maximum systemic circulation concentrations of tasonermin were reached 5 min after the start of ILP and are approximately 200 times less than in the perfusion circuit. Under conditions of greater than 2% systemic leakage (observed in 13 of 51 patients) maximum systemic concentrations of tasonermin were still at least ten times lower than in the perfusion circuit.

The toxicological profile of tasonermin has been investigated in preclinical studies using mice, rats, rabbits, dogs and monkeys. Haematological and circulatory changes, decreased well-being and weight gain as well as alterations in the function of liver and kidneys were the main adverse effects observed on repeated tasonermin administration. The haematological changes included anaemia, increased haematocrit and increased or decreased leukocytes and platelets depending upon species and treatment duration. The circulatory changes included decreased blood pressure and, in some studies, increased heart rate and decreased contractility. The synthesis capacity of the liver was lowered as indicated by increased liver enzymes. Altered renal function comprised increased water and sodium excretion as well as increased urea and creatinine. No NOTEL (No Observed Toxic Effect Level) could be established in the preclinical studies with the exception of a 7-day administration of 0.1 µg/kg in monkeys. The changes observed at the low dose of the 13-week studies can be classified as minimal and fully reversible.

Tasonermin does not cross the intact blood-brain barrier to a significant extent in mice. In the Rhesus monkey, whole body radiography following administration of radiolabelled tasonermin indicated no specific distribution pattern. Tasonermin did not cross the placenta or pass into necrotic tumour. In the Rhesus monkey, pharmacokinetic studies following intravenous injection of tasonermin indicated a non-specific, non-saturable excretion via glomerular filtration in the kidney. A second specific and saturable elimination mechanism involving tasonermin receptors seems likely.

No evidence has been found of any mutagenic effect, neither in vivo nor in vitro. No reproduction toxicity or carcinogenicity studies were performed due to testing being inappropriate as the intended clinical use of tasonermin is in ILP for soft tissue sarcoma treatment.

To cover the intended clinical use of tasonermin, ILP experiments were performed in hind legs of healthy rats using different doses in the same tasonermin concentration as in the clinical situation in the human. Except for slight aggravation of ischaemic effects in higher doses, standard histological examinations of the skin, muscle, bone, nerves and vessels revealed no difference in findings between tasonermin-treated and control animals. No late detrimental effects of tasonermin were seen.