In pharmacology, a substance used in the treatment of a disease is by definition called a drug . Instead of treating a disease, a drug may in some cases be toxic due to numerous adverse effects. Some of them are side effects that occur when the chemical compound of the drug binds to its intended receptor though at an inappropriate concentration or in the incorrect tissue, whereas others are a result of the interaction of the drug with unintended targets. In order to overcome these two types of adverse effects, drug delivery systems have been developed to safely transport a medicinal substance, so that it exerts its therapeutic effect . The two main branches of drug delivery systems that we will discuss are drug targeting and controlled release.
Drug targeting systems make sure that the therapeutic agent interacts with a specific target receptor in the right tissue. The main benefit of these techniques is that they minimize side effects by avoiding interaction between the drug and unintended targets. There are two main types of drug targeting are passive and active targeting. The former aims to accumulate the drug in a selected area in the human body, whereas the latter to achieve a ligand-receptor type of interaction between the drug and a distinct molecule of the targeted cells. The second type, which is in fact the most difficult system to engineer, has three phases. First, the medicine is delivered to the targeted site (organ or tissue). Second, the drug heads to the selected types of cells such as cancer cells. Finally, the drug interacts with a specific site (receptor), which can be extracellular or intracellular. The drug carriers play a special role in drug targeting. They are the vehicles that transfer the drug through each phase of the active drug targeting. The ideal drug carriers should be non-toxic, biodegradable, should recognize the target cell and release the drug upon recognition .
The second branch of delivery systems – controlled drug release – achieves an even concentration of the drug as opposed to conventional drug administration methods that may result in fluctuations in the concentration of the drug. In Figure 1, the vertical axis depicts the concentration of the drug and its levels of ineffectiveness, toxicity, and therapeutic action and the horizontal axis depicts the time scale. The fluctuations in the concentration of a conventionally administered drug are shown with a continuous line, while the even concentration of a drug administered through a controlled release system is shown with a dotted line .
Therefore, the goal of controlled release drug delivery systems is to administer an optimal quantity of medicine through a minimum number of administrations and to release a specific amount of drug over a desirable length of time.
There are a variety of controlled release systems which are classified according to the mechanism they utilize in order to release the drug in the organism at a controlled rate. Some of these techniques involve dissolving or dispersing the drug in a polymeric matrix that starts to diffuse after the administration, while simultaneously releasing the drug. The release rate of the drugs in this case is a function of time and the mass of the non-diffused matrix. In other cases, the drug is enclosed in a core surrounded by a polymer membrane. The drug is released through the pores in the surface of the membrane or after the biodegradation of the polymer membrane or through a small opening due to the increase in pressure from water molecules that move in the semi-permeable membrane. There are even some delivery systems in which the release of medicine can be controlled and enhanced by using external stimulation like exposing the drug carriers to ultrasounds or applying a magnetic field and others in which the release is triggered by an alteration of environmental conditions such as temperature, type of solvent, pH or concentration of a specific substance .
Designing a drug delivery system is a very complex procedure. There are numerous obstacles that must be surmounted in order for a drug delivery system to be functional and not pose any threat to the health of the patient. Some aspects that must be considered are the properties of the drug, side effects, the path that the drug delivery system follows, the target, and the disease. Therefore, the products produced for drug delivery purposes have to be biochemically inert, non-immunogenic, physically and chemically stable in the conditions of the human body and their size must belong to the micro or even nano scale. An example of the range of this scale is the range between the diameter of a strand of human hair and the diameter of a DNA molecule . Drug delivery systems should also transfer the ideal dosage of medicine to the targeted site and in the preferred amount of time with the minimum loss of drug . Furthermore, as with any other pharmaceutical innovation, there are ethical and economical concerns about the availability of drug delivery systems, since pharmaceutical industries invest a great amount of funds for their research and design.
Drug delivery is a promising method with a great potential to become an effective and powerful tool in pharmacology. It is however, a relatively new approach to disease treatment and a lot more research needs to be done. Scientists aim to personalize drug delivery to the patient, apply it to a wider range of diseases and make the techniques needle-free and more patient friendly in general . Moreover, in the future scientists hope to use drug delivery techniques in gene therapy, the therapy of hereditary diseases by transferring the preferred genetic material in the nucleus of the cells . To accomplish all of the above researchers need to gain deeper insights into the cellular and molecular processes generating each disease through a better understanding of molecular biology and molecular medicine . Years of thorough and systematic research can in other words render drug delivery as the solution to many currently incurable diseases and other unsolved medicinal problems.
Taking everything into consideration, the consumption of medicine for therapeutic purposes should be like, as the title implies, waging a war against the cause of a disease. Herein comes the full potential of drug delivery systems that in a sense, reflects an ancient Chinese proverb stating: “Know yourself, know your enemy. A thousand battles, a thousand victories.”
By relying on the knowledge of both the basic characteristics of the drug and the disease on a molecular level, drug delivery systems are in other words, tailor-made methods of administration capable of improving the efficacy of current treatments and offering new medicinal benefits.
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Dimitris Bratsios is a second year student at the Aristotle University of Thessaloniki majoring in Chemical Engineering. He is interested in the field of biomedical engineering, especially in tissue engineering, biomaterials and drug delivery.