Blood coagulation is a process that changes circulating substances within the blood into an insoluble gel. The gel plugs leaks in blood vessels and stops the loss of blood. The process requires coagulation factors, calcium and phospholipids.

•             The coagulation factors (proteins) are manufactured by the liver.

•             Ionized calcium ( Ca++ ) is available in the blood and from intracellular sources.

•             Phospholipids are prominent components of cellular and platelet membranes. They provide a surface upon which the chemical reactions of coagulation can take place.

Coagulation can be initiated by either of two distinct pathways.

•             The Intrinsic pathway can be initiated by events that take place within the lumen of blood vessels. The Intrinsic pathway requires only elements (clotting factors, Ca++, platelet surface etc.) found within, or intrinsic to the vascular system.

•             The Extrinsic pathway is the other route to coagulation. It requires Tissue Factor (tissue thromboplastin), a substance which is "extrinsic to", or not normally circulating in the vessel. Tissue Factor is released when the vessel wall is ruptured.

Regardless of whether the Extrinsic or Intrinsic pathway starts coagulation, completion of the process follows a common pathway. The common pathway involves the activation of factors: X, V, II, XIII and I. Both pathways are required for normal hemostasis and there are positive feedback loops between the two pathways that amplify reactions to produce enough fibrin to form a lifesaving plug. Deficiencies or abnormalities in any one factor can slow the overall process, increasing the risk of hemorrhage.

The coagulation factors are numbered in the order of their discovery. There are 13 numerals but only 12 factors. Factor VI was subsequently found to be part of another factor. The following are coagulation factors and their common names:

•             Factor I - fibrinogen

•             Factor II - prothrombin

•             Factor III - tissue thromboplastin (tissue factor)

•             Factor IV - ionized calcium ( Ca++ )

•             Factor V - labile factor or proaccelerin

•             Factor VI - unassigned

•             Factor VII - stable factor or proconvertin

•             Factor VIII - antihemophilic factor

•             Factor IX - plasma thromboplastin component, Christmas factor

•             Factor X - Stuart-Prower factor

•             Factor XI - plasma thromboplastin antecedent

•             Factor XII - Hageman factor

•             Factor XIII - fibrin-stabilizing factor

The liver must be able to use Vitamin K to produce Factors II, VII, IX, and X. Dietary vitamin K is widely available from plant and animal sources. It is also produced by normal intestinal flora. A deficiency is rare but may occur:

•             in newborns because they must first develop normal flora to produce Vitamin K, or

•             when the flora is disturbed by broad-spectrum antibiotics.

At birth and throughout childhood, Factor VIII levels are the same as adult values. Many other factor levels are below adult levels at birth, some as low as 10% of adult levels. Theses levels increase toward the adult levels by age 6 months, although they may remain mildly below adult normal range throughout childhood. Despite lower levels, newborns and children do not normally experience bleeding. This confers some level of antithrombolic protection in youth. During pregnancy Factor XI can decrease, but fibrinogen and factor VIII increase.

The clotting factors are Factor I (fibrinogen), Factor II (prothrombin), Factor III (tissue thromboplastin or tissue factor), Factor IV (ionized calcium), Factor V (labile factor or proaccelerin), Factor VII (stable factor or proconvertin), and Factor VIII (antihemophilic factor). Additionally, the coagulation factors also include Factor IX (plasma thromboplastin component or the Christmas factor), Factor X (Stuart-Prower factor), Factor XI (plasma thromboplastin antecedent), Factor XII (Hageman factor), and Factor XIII (fibrin-stabilizing factor).

The liver uses vitamin K to produce some of the factors such as Factors II, VII, IX, and X. Normally, vitamin K can be consumed through the diet from plant and animal sources. The normal flora of the intestine also produces vitamin K.

The nonclinical pharmacology assessment in relevant animal models provides critical PK and PD data to support first-in-human (FIH) clinical trials. PK/PD parameters can be included in the toxicology program or can be conducted as stand-alone studies. Bioavailability studies may be needed if the biotherapeutic is administered subcutaneously (SC) and biodistribution may need to be assessed if the distribution of the drug is not solely in blood.

For some of the coagulation factors it was directly shown that they have a linear PK profile and that scaling can be applied between animal species and humans . It is important to consider that the distribution volume of large intravenously injected proteins like rFVIII is mainly limited to blood. For coagulation factors rFVIII or rFIX, activity of the drug can be measured by standard in vitro assays, including prothrombin time (PT) or activated partial thromboplastin time (aPTT) available as commercial assay kits (e.g. APTT-SP™, STA-PTT™, SynthaFax™, QuikCoag™, Dade Actin™, and APTTSA™). These assays can be utilized to measure not only PK but also PD of the administered factor as their activity rather than concentration in animal and human blood is the most relevant parameter. This approach allows for direct correlation of the exposure to PD activity in vivo

Since coagulation factor therapy has been established for many years, first with plasma-derived then with recombinant products, great efforts to standardize these human replacement products have been made. As a result, reference standards for FVIII and FIX are provided by the World Health Organization (WHO) to provide standardization of the activity of coagulation factor products made by different upon their administration in humans (www.who.int/bloodproducts/ivd/coagulation_disorders/en/). On the web page the WHO defines their standardization:

The WHO International Standards (IS) for the measurement of blood coagulation factors and inhibitors in plasma are used by manufacturers and reference laboratories to calibrate their secondary standards. This assures a correct diagnosis of clotting factor deficiencies and monitoring of treatment with certain plasma products such as in patients with haemophilia. The unitage assignment of clotting factors and inhibitors takes into account an external reference point, namely the concentration in 1 ml of “average” fresh human normal plasma.

With Regards,
John Mathews
Managing Editor
Journal of Phlebology and Lymphology