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Delivery Of Proteins: Target Site-specificDelivery By The Parenteral Route

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• There are three different ways in which cells can communicate with each other:

- Endocrine, paracrine, and autocrine pathway

• Endogenous agents such as insulin

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• the presence of these mediators may activate a complex cascade of events that needs to be carefully controlled.

• Therefore, key issues for their therapeutic success are: (i) access to target cells, (ii) retention at the target site, and (iii)proper timing of delivery

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Approaches For Rate-controlled Delivery

• Chemical approaches can be used to change protein characteristics. Polyoxyethylene glycol attachment to proteins changes their circulation half-life in blood dramatically.

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• The active compound never reaches the target site, because it is rapidly eliminated intact from the body through the kidneys, or it is inactivated through metabolic action (e.g., in the liver).

• Only a small fraction of the drug reaches the target site. By far the largest fraction of the drug is distributed over non-target organs, where they exert side effects; in other words, accumulation of the drug at the target site is the exception and not the rule

Site-specific Delivery (Targeting) OfProtein Drugs

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• Many drug molecules (in particular high MW and hydrophilic molecules) do not enter cells easily. This poses a problem if intracellular delivery is required for their therapeutic activity.

• Attempts are made to increase the therapeutic index of drugs through drug targeting:

• (1) by specific delivery of the active compound to itssite of action,• (2) to keep it there until it has been inactivated anddetoxified

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Anatomical, Physiological, and PathologicalConsiderations Relevant for Protein Targeting

• Carrier-mediated transport in the body depends on the physicochemical properties of the carrier: its charge, molecular weight/size, surface hydrophobicity, and the presence of ligands for interaction with surface receptors .

• If a drug enters the circulation and the target site is outside the blood circulation, the drug has to pass through the endothelial barrier.

It should not be considered as one big pool without internal barriers for transport.

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Soluble Carrier Systems for TargetedDelivery of Proteins

• Antibodies are “natural targeting devices.”• MAb can affect the target cell function upon attachment.

Complement can be bound via the Fc receptor and subsequently cause lysis of the target cell.

• Alternatively, certain Fc receptor-bearing killer cells can induce “antibody dependent, cell-mediated cyto-toxicity” (ADCC), or contact with macrophages can be established.

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• A problem that occurs when using murine antibodies for therapy is the production of human anti-mouse antibodies (HAMA) after administration.

• Solutions:• Concurrent administration of immunosuppressive agents .Humanization of Mab1- Completely human MAb can be produced by transfecting human antibody genes into mouse cells, alternatively, transgenic mice can be used

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2- Chimeric (partly human, partly murine) molecules consisting of a human Fc part and a murine Fab part, with the antigen binding sites or, alternatively, only the six complementarity determining regions (CDRs) of the murine antibody can be grafted in a human antibody structure. CDR grafting minimizes the exposure to murine material.