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Minireview Magnetically controlled targeted micro-carrier systems
Life Sciences, Vol. 44, pp. 175-186 Printed in the U.S.A.
Pergamon Press
MINIREVIEW
MAGNETICALLY CONTROLLED TARGETED MICRO-CARRIER SYSTEMS P.K. Gupta* and C.T. Hung§ Department of Pharmacy, University of Otago, Dunedin, New Zealand
Summary Magnetically controlled targeted drug delivery systems are aimed at concentrating drugs at a defined target site, with the aid of a magnetic field. This technique has been developed specifically for directing drugs away from the reticuloendothelial system (RES). Literature on this topic suggests that these delivery systems are capable of altering the distribution of chemotherapeutic agents in the body. Hence these delivery devices offer the possibility of improving the therapeutic efficacy of the associated drugs. This paper reviews the work done to date towards the development and evaluation of biodegradable and non-biodegradable magnetic targeted drug delivery systems and outlines their future prospects and limitations in cancer chemotherapy. Cytotoxic agents have extensively been employed in cancer chemotherapy since the early 1940's (1). However, the non-specificity of these drugs towards cancer and healthy cells of the body has curtailed their effective use in chemotherapy. Several modaiities have been tried to improve the therapeutic efficacy of these agents. These include multiple chemotherapy, regional drug delivery, long-term low-dose infusion, or short-term high-dose infusion of cytotoxic agents (2-12). However there is little quantitative evidence regarding the enhanced therapeutic efficacy of these dosing strategies over the conventional dosage regimens (13-15). The lack of specificity of these compounds has led scientists towards developing delivery systems which may improve their turnour : non-tumour tissue distribution in the body, so that the cumulative toxicities intrinsically associated with them are reduced. Targeted drug delivery systems are aimed at increasing the concentration and duration of action of drugs at the tumour site so that the survival of healthy cells is not considerably affected. The concept of magnetically controlled drug targeting probably emerged in 1970's when Zimmermann and Pilwat (16), and-Widder et al.(17,18) developed magnetic erythrocytes and magnetic albumin microspheres respectively, for the delivery of cytotoxic drugs. The unique feature of these delivery systems over other drug targeting techniques is their ability to minimise the carrier uptake by RES (19). Conceptually, the intended drug and a suitable magnetically active component (e.g., Fe304) are formulated into a pharmaceutically stable system. This is injected through the artery nourishing the tumour tissue in the presence of an external magnet which can generate sufficient field strength and gradient to confine the carrier at the target site. The magnetic field is directed exclusively onto the organ or tissue residing turnout cells for a sufficient period to allow the transfer of the carrier from the blood compartment to the extravascular target tissue. The drug released from the carrier in a controlled manner then exerts its pharmacological action at the cellular * To whom all correspondence should be addressed at : College of Pharmacy, University of Kentucky, Rose street, Lexington, Kentucky, USA. § Present address: Zenith Technology Corporation Limited, 156 Fredrick Street, P O Box 1777, Dunedin, New Zealand. 0024-3205/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc
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and/or sub-cellularlevelof the turnout tissuewithout criticallyaffectingthe survivalof normal tissue (19-21). This is an attractivemethod of drug localization,provided that the targeted tissue has abundant vascular supply and is accessible to magnetic fields. Investigationscarriedout using magnetic carrierhave demonstrated 10 to 100 fold increase in the therapeuticindex of the associateddrug (18,19,22,23).So far similarincrease in the therapeuticindex of chemotherapeutic agents have not been demonstrated by any other drug targetingdevice.
One of the major limitations of most drug delivery systems in cancer chemotherapy is their inability to extravasate the endothelium of the tumour tissue. Hence the included drug may not be available exclusively at the cellular and/or sub-cellular level (19,24). However it has been shown that magnetic drug targeting allows extravasation of carder and increases the probability of sub-cellular or third-order drug targeting (18,25-27). Slow release of an encapsulated cytotoxic agent prevents endothelial injury of the target tissue, which is usually observed following their administration as an infusion (28,29). The process of drug localization using magnetic delivery systems is based on the competition between forces exerted on the particles by blood compartment, and magnetic forces generated from the applied magnet. When the magnetic forces exceed the linear blood flow rates in arteries (10 cngsec) or capillaries (0.05 cm/sec), the microspheres (-1 ~tm) are retained at the target site and may be internalized by the endothelial cells of the target tissue (18,30,31). It has been suggested that at the arterio-capillary blood flow rate of 0.05 to 0.1 cm/sec, 20% w/w of magnetite is sufficient to achieve 100% retention of the magnetic carder using a 8000 Gauss magnet (30). Contrary to the drug targeting system discussed above, Kato (24) have developed large (200-300 gm) non-biodegradable magnetic microcapsules for drug targeting at the arterial level. Despite the fact that these particles do not extravasate the vascular endothelium of tumour tissue, and their ultimate fate in the body is not clearly understood, this carder has demonstrated promising results in experimental investigations (24,32). Table I shows a general classification of magnetically controlled targeted drug delivery systems, each of which is discussed in the subsequent part of this review. Table I Classification of magnetically controlled targeted drug delivery systems Class
Example(s)
A. BIODEGRADABLE 1. Particulate carders
Magnetic emulsions Magnetic starch microspheres Magnetic poly(alkyl cyanoacrylate) nanoparticles Magnetic albumin microspheres Magnetic erythrocytes Magnetic liposomes
2. Vesicular carders B. NON-BIODEGRADABLE 1. Particulate carriers
Magnetic ethylceUulose microcapsules
Magnetic emulsions All magnetic emulsions reported so far are of o/w type (33-36). The dispersed phase of these emulsions consists of a suspension of magnetite in oleic acid or ethyl-oleate,
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with caesin as the emulsifier. The effect of magnetic field strength (0, 2000, 4000 or 6000 Gauss) and field application time (10 or 60 rain) on the localization of magnetic ethyl-oleate emulsion, containing 14C-palmitic acid (36) or methyl-CCNU (35), to the lungs of rat has been investigated. Ten rain after administration of the emulsion through the tail vein, the radioactivity in lungs was 2 times higher in the presence of a magnet, compared to the control where no magnet was used. This level was 6 to 7 times higher than those analysed in liver, spleen, kidney and heart. However, 60 rain after administration, the radioactivity in lungs reduced appreciably, with concurrent increase in the radioactivity of liver and heart. It was suggested that the emulsion is washed from the lungs as a function of time (36). The data obtained at these two time points did not justify the use of higher magnetic fields for increasing the effectiveness of targeting. However, upon using the emulsion containing methyl-CCNU, remarkable increase in lung drug levels was noticed with an increase in magnetic field strengths (35). But no information was provided regarding the disposition of magnetic emulsion as a function of time. The possible reasons for the inefficiency of this delivery system in drug targeting include: (i) low magnetic susceptibility resulting into their poor magnetic drag and hence particle extravasation; (ii) short half-life of drug release due to large surface to volume ratio of the formulation; and (iii) marginal physical stability. Maunetic Starch Microsphere~ The use of magnetic starch microspheres in d~U¢l~ targeting was suggested by Mosbach and co-workers (37-40). Upon administration of 5I-albumin coupled magnetic starch microspheres (2-10 Ixm) through the marginal ear vein of rabbits, 4 to 8 fold increa in radioactivity in the ear, exposed to a 7000 Gauss for 10 rain, was demonstrated. Despite the fact that the animals were sacrificed immediately after removing from the magnetic field, 80% of the injected radioactivity was detected in the lungs. The large size of the microspheres accounted for this undesirable or negative targeting of the radioactivity. These studies also revealed in vivo de-iodination of the carrier. Potential use of this carrier in drug targeting requires further investigation. Magnetic nolv(alkvlcvanoacrvlate~ Nanooarticles The use of magnetic poly(isobutylcyanoacrylate) nanoparticles in circumventing the uptake of carrier by the liver has been suggested by Ibrahim et al. (41). In an in vitro experiment, these workers demonstrated that 28% w/w of magnetite in 0.22 gm nanoparticles is necessary for their effective targeting. Acute toxicity studies in mice showed that the LD50 of magnetic poly(isobutylcyanoacrylate)nanoparticles is equivalent to that of non-magnetic nanoparticles (i.e., 245 mg/kg). This study also confLrmed the non-toxicity of magnetite particles up to the doses of 1500 mg/kg. Upon intravenous administration of 3H-dactinomycin adsorbed magnetic nanoparticles, three fold increase in targeting to the left kidney of mice, exposed to a 8500 Gauss magnet for 10 rain, was observed in comparison to the right kidney which served as a control. Another experiment carried out with each kidney of mice exposecl to a 8500 Gauss magnet for 10 rain revealed three times higher radioactivity in kidneys and a third lower radioactivity in liver, compared to animals which were not exposed to magnets. However, in both experiments all the data were collected at only 10 rain after dosing and no information was provided on drug distribution to other tissues (41). It is emphasised that due to their toxic nature (42,43), poly(alkylcyanoacrylate) particles appear to possess low probability of regulatory approval. Like magnetic emulsions, these formulations also exhibit short half-life of drug release due to the presence of major drug load as an adsorbed fraction (44). Ma~etic Albumin Microsoheres Magnetic albumin micmspheres are micrometer size particles, routinely prepared by heat or chemical hardening of a w/o emulsion, consisting of drug, magnetite and albumin as dispersed droplets in a suitable oil (19,26,45-48). Studies investigating their physico-ehemical characteristics have revealed that the in vitro release of a cytotoxic drug
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from these particles can be controlled (48). However these studies have also demonstrated considerable batch to batch variation in the particle size and drug content of the magnetic microspheres (48). The within and between batch homogenity of these particles with respect to their drug and magnetite content is still unknown. The first in vivo model for verifying the drug targeting efficacy of magnetic albumin microspheres was proposed by Widder et al. (17,18). Rat's tail was used as a target organ and was demarcated into four equal regions (3.5 cm), called T1, T2, T3 and T4 respectively from the base of the tail. T1 was considered as the dose-administration site and T3 as the target-site. The magnetic drug carrier was infused intra-arteriaUy in a controlled manner so as to obtain a homogeneous capillary-level distribution of the particles within the magnetic targeting volume. In one experiment, the animals were sacrificed 30 rain after dosing with 0.5 mg of 125I-magnetic albumin microspheres in presence of a magnetic field applied for 30 rain (field strength 0, 4000, 6000 or 8000 Gauss). Although increase in magnetic field strength did result in a higher percentage of radioactivity at T3, the number of microspheres localized in liver, lungs and spleen did not decrease to any considerable extent. Shunting of blood vessels around the target region was suggested as the cause of microspheres pooling in the RES. Interestingly, the animals sacrificed at 24 hr after dosing also demonstrated high localization of radioactivity at T3. It was shown that the microspheres at T3 pass from the vascular compartment into the interstitial tissue, preventing their delocalization after removing the magnet (18). To verify the effect of magnetic field application time on the in vivo distribution of low-dose adriamycin, Senyei et al. (22). administered 0.05 mg/kg of 125I-magnetic albumin microspheres in rats, with the T3 region exposed to a field of 8000 G for 5 to 60 rain. The animals were sacrificed immediately after removing the field and various tissues analysed for 125I-counts. Administration of equal dose of microspheres in the absence of magnetic field served as a control. Regardless of the field application time, this study demonstrated 7 to 10 times higher radioactivity at T3, compared to those at liver, lung or spleen. The control animals demonstrated no radioactivity in tail segments. In addition these animals indicated twice the radioactivity in liver, lung and spleen than the treatment groups (22). Since the animals were sacrificed immediately after removing the magnetic field, this study could not identify the effect of a field application time on the actual disposition of the magnetic albumin microspheres, several hours after removing the field. However this study revealed that at a 60 rain post-administration point, a 0.05 mg/kg microsphere dose produced target-site adriamycin concentrations twice as high as that resulted from 5 mg/kg intravenous solution dose (22). The efficacy of magnetic albumin microspheres in the tumour bearing animals have also been investigated (49,50). Rats transplanted with Yoshida sarcoma at T3 were administered with a single dose of 0.5 mg/kg adriamycin in the form of magnetic albumin microspheres, in presence of 5500 Gauss magnetic field applied for 30 min post-dosing. Results were compared with six controlled experiments: (i) untreated animals; (ii) 5 mg/kg free adriamycin given intravenously; (iii) 5 mg/kg free adriamycin given intra-arterially; (iv) 0.5 mg/kg free adriamycin given intra-arterially; (v) equivalent dose of non-drug bearing magnetic albumin microspheres administered in presence of a magnet; and (vi) 0.5 mg/kg adriamycin as magnetic albumin microspheres given in absence of magnet. Whereas 80 to 100% of the animals in the controlled groups died between day 13 and 16 after tumour inoculation, 100% of the animals survived for up to 30 days (treatment time) in the group treated with 0.5 mg/kg adriamyein via magnetic carder in presence of magnet. In addition, 75% of the animals (9/12) in this group exhibited complete remission of tumour and another 17% (2/12) showed def'mite tumour regression (50). One animal showed no change in tumour size during the treatment time (30 days). These results were verified in another study (51), where 0.5 or 2.5 mg/kg adriamycin was administered using the magnetic carrier, in presence of a 5500 Gauss magnet. This study revealed no significant difference between the two adriamycin doses, when given as magnetic carrier for the treatment of tumour bearing animals (51).
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In a separate study where drug-free magnetic albumin microspheres were targeted to Yoshida sarcoma in rat's tail, Widder et al. (25) demonstrated extravasation of microspheres as early as 10 rain after their administration. The microspheres were shown to be endocytosed by tumour. However the actual uptake of microspheres occurred in a very small fraction of ceils. Using adriamycin-associated magnetic albumin microspheres in healthy animals, second-order drug targeting has been confLrrned in our laboratory (26). Based on these results it is evident that this therapy could be beneficial for drugs discarded in cancer chemotherapy due to their poor cell uptake. An extensive study on the effect of magnet type on the targeting efficiency of magnetic albumin microspheres to rat's tail, and their localization to other organs, has been conducted by Ovadia et al. (52). This study proved that a custom-made bar magnet (field strength 4200 Gauss; gradient at pole face: 2700 Gauss/cm) was as effective as a horse-shoe magnet (strength 7000 Gauss; gradient 4000 Gauss/era), in localizing the magnetic carrier at the tail target site (T3). These results led to the conclusion that the magnetic field gradient, rather than strength, is important in the retention of microspheres. It also was suggested that the magnetic responsiveness of these particles may achieve saturation at field strengths greater than 4000 Gauss (52). In addition, this study showed that the complexity of target site vasculature, and its depth and permeability, may restrict the application of magnetic albumin microspheres in localized drug therapy. It was concluded that to achieve maximum targeting efficiency, the magnetic carrier should be administered closest to the applied magnetic field (52). In a study comparing the size of the magnetic carrier (1 versus 3 ~tm) on its localization to lungs, in presence of a 3000 Gauss magnet, use of 3 ~tm microspheres demonstrated superior results than the smaller carrier (53). In a subsequent study, these workers (43) monitored the localization of 125I-magnetic albumin microspheres to the lungs and the left kidney of mice and rats, respectively. In both experiments, 3000 Gauss magnets were applied for 10 or 60 rain post-dosing, and the radioactivity at the target-site was monitored at 10 as well as 60 rain after dosing. In mice, when the magnets were used throughout the experiment, the lung radioactivity increased from 17% of the injected dose at 10 rain to 28.2% at 60 min. However when the field was applied for only 10 min, the radioactivity at 60 rain decreased substantially as compared to that at 10 rain (54). In rat kidney, when the field was applied throughout the experiment, no increase in the localization of the radioactivity was noticed at 60 rain, as compared to that at 10 min. But when the field was applied only for 10 rain, the radioactivity decreased from 56.4% of the injected dose at 10 rain to 25.5% at 60 min, with simultaneous increase in liver radioactivity. The reduction in targeting efficiency as a function of time was suggested as a result of the redistribution of microspheres in the absence of a magnetic field. It was therefore proposed that the magnetic field application time should be prolonged to facilitate the retention of microspheres at the target site (54). The larger microspheres (e.g. 3 Ixm) localize upstream in the vessels where they encounter greater resistance for the extravasation due to thicker epithelium of the vessel wall. It is also emphasised that due to-the application of a magnetic field, the microspheres tend to aggregate and cluster into longitudinally aligned short cylindrical chains (31), and this process is rarely affected by the blood flow. Due to this phenomenon, the functional diameter of microspheres in magnetic field is significantly greater than the actual diameter observed under an electron microscope. In a study carried out on rats with AH 7974 lung metastases (55), use of adriamycin-associated magnetic albumin microspheres, in the presence of 6000 Gauss magnet applied for 10 rain, demonstrated increased survival of animals compared to the untreated animals, or animals treated with the free drug. A study by Bartlett et al. (56) in the canine osteosarcoma of a limb of dogs demonstrated little difference between the routes of administration and infusion pattern of the suspension of a radioactive magnetic albumin microspheres, towards the retention of the particles in the tumour tissue, 15 min after the removal of an electromagnet from the target region. However this study revealed that the localization of microspheres occurs
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only in the weU-perfused and soft tissue regions of the tumour. Insignificant radioactivity was traced in the necrotic or the osseous portions of the tumour. Using magnetic albumin microspheres containing 10% w/w of vindesine sulphate, Morris et al. (57) have demonstrated 85% total remission and almost 95% regression in Yoshida sarcoma implanted in rat tail. This study also confirmed no significant difference between the 0.5 and 2.5 mg/kg doses of vindesine for the eradication of sarcoma, when delivered via the magnetic carrier. The control animals receiving either free or non-rnagnetieaUy targeted drug-loaded microspheres displayed widespread metastases and ultimate death (57). In an effort to modulate localized inflammation, Ranney (27) has recently demonstrated successful targeted delivery of N-formyl methionylleueyl phenylalanine to rat lungs via magnetic albumin microspheres, in presence of a 5500 Gauss thoracic magnet. The targeted mierospheres were observed in the extravascular compartment as early as 5 rain post-administration. The targeted bio-modulator induced local accumulation of neutrophils and acute tissue injury due to chemotaxis. A study investigating the multiple tissue disposition of adriamycin, intra-arterially targeted to rat tail via the magnetic albumin microspheres, in presence of a 8000 Gauss magnet applied for 30 rain, revealed 64% higher exposure of the target-site compared to the equivalent dose administered as a free drug (see Fig. 1) (46). Although most other tissues, including heart, demonstrated upto 75% reduction in their exposure towards the magnetically targeted adriamycin, liver uptake of the carrier could not be prevented (46). 20
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Time (Hrs) Fig. 1 Log-linear plots of tissue concentration versus time for the tail target-site (A) and liver (B), following the administration of 2.0 mg/kg adriamycin HC1 as a solution (r~) or via the magnetic albumin microspheres, in presence of a 8000 Gauss electromagnet applied for 30 min post-dosing (e). Each point represent mean of three rats. (Adapted in part from reference 46). .Magnetic Ervthrocvtes The clinical use of cell transfusions has led to the use of erythrocytes in drug delivery and targeting. As well as being biodegradable even after chemical treatment, use of a patient's own erythrocytes precludes the possibility of immunological responses often encountered with other exogenous drug carriers (58-62). Zimmermann and Pilwat (16) proposed the use of magnetic erythrocytes in site-specific delivery of drugs. According to these workers, magnetic particles (10-20 nm) can easily be incorporated in erythrocyte ghosts at an electric field intensity of 15 KV/cm and a pulse length of 50 I.tsec, in isotonic solutions of sucrose, glucose or mannitol (63). It has been claimed that the targeting
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efficiency of these carriers can be as high as 70% and their release profiles can be controlled by adjusting the stability of cell membranes or by electric lysis of cell in vivo (63-65). However the premature release of drug due to inadequate membrane stability and poor penetration of this carrier through tumour endothelium (19,66) have curtailed its pharmaceutical scale-up and further experimental evaluation in drug targeting. Magnetic Lipo~omes Liposomes is a particular class of drug carrier which has been studied more extensively than any other existing targeting drug delivery system. These consist of one or more concentric bilayer structure in a fashion that the lipid layers alternate with the aqueous layers (66-71). They exist in a size range of a few nanometers to several microns. A unique feature associated with these carders is their ability to incorporate both water soluble and oil soluble therapeutic agents in the aqueous and the lipid layers, respectively. It has been claimed that judicious choice of lipid composition, size, surface charge and surface groups may control the in vivo fate of drugs incorporated in liposomes (67,72,73). However the entrapment capacity of most liposomal preparations is < 5% w/w, and this carrier is rapidly cleared by RES. Increased popularity and successful experimental application of magnetic drug targeting has led to the development of magnetically responsive liposomes. K.iwada et al. (74) has demonstrated meagre but significant increase in the localisation of 3H-inulin delivered via this carrier in Yoshida sarcoma implanted in foot pad of rats. It was concluded that increase in magnetic field strength may increase the turnout versus non-tumour tissue distribution of drugs administered via this carrier. In view of the poor stability of liposomes, it is rather difficuk to predict its future prospects in drug targeting. Magnetic Ethvlcellulose Microcansules Transcatheter vessel occlusion for the management of hemorrhage, spenomegaly and radiotherapy of carcinomas has been practiced since last two decades (75-78). This technique is still popular for inducing temporary hypoxia and altering the distribution of chemotherapeutic agents (79-84). The concept of embolization, followed by chemotherapy of carcinomas has been successfully promoted by Kato et al. (85-88) and the technique has been named as arterial chemoembolization. This method involves infusion of drug-loaded microcapsules after arterial catheterization of the tumour tissue (24,89,90). Due to their large size (200-300 ~tm), these particles along with the drug are localised near the target site immediately after their administration. This leads to ischemia and anoxia of tumour tissue. In addition, sustained release of drug in high concentration increases its contact time with tumour cells (24). This approach is said to preclude the necessity of carrier extravasation. It is believed that intrinsic permeability of tumour tissue or increase in tissue permeability due to anoxia, assists rapid cellular action of the associated drug (24). Despite its clinical success, arterial chemoembolization has been found to be non-practicle for drug targeting in carcinomas with complicated vascular supply. In addition, existence of arterio-venous fistula in tumours may reduce the probability of carrier occlusion at the tumour site (24). To overcome these problems, ethylcellulose microcapsules with 16% w/w zinc ferrite and 50% w/w mitomycin C, in presence of a suitable magnet, have been tried. This technique is known as magnetic arterial chemoembolization (32,91-94). In an experiment utilizing rabbits with bladder tumour, use of drug-free magnetic microcapsules in presence of magnet, or drug-loaded microcapsules in absence of a magnet, displayed no response towards tumour eradication. However, use of drug-loaded magnetic microcapsules in presence of a magnet (3500 Gauss/era) demonstrated successful necrosis of the tumour (32). In subsequent experiments on rabbits with transplanted VX2 carcinoma in hind limb, intra-arterial administration of mitomycin C via magnetic microcapsules, in presence of a 1250 Gauss magnet applied for 30 rnin, displayed marked reduction in tumour size over a period of 40 days. The animals receiving free drug or no drug showed 12 fold increase in tumour size during the same period, as compared to the treatment group (95). However some animals died in the treatment group, due to infection following the necrosis of the foot.
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The feasibility of magnetic microcapsules in intra-vesicular administration of drugs has also been investigated (96,97). In one study, it was shown that intra-vesicular administration of free mitomycin C (control i) or placebo microcapsules plus free drug (control ii) resulted in no detectable drug concentrations in the urinary bladder. However administration of mitomycin C loaded magnetic microcapsules, in presence of a 2400 Gauss magnet fixed onto the external wall of urinary bladder, resulted in significant amount of drug levels in the target tissue. Nonetheless, this group also demonstrated 4 to 6 times higher drug levels in blood compared to the controlled groups (96,97). Despite several encouraging reports, the concept of magnetic arterial embolization has failed to receive the anticipated impact mainly due to the large size and non-biodegradability of the carrier. Inability of this carder to extravasate from the embolizing vessel precludes the possibility of effective chemotherapy, as demonstrated with magnetic albumin rnicrospheres at the intracellular level (18,26,27). Long term occlusion of tumour tissue vasculature may favour the formation of thrombus, and affect the activity of associated healthy tissues. Occlusion of a blood vessel by the carder means that all the released drug may not necessarily be available for cytotoxic action at the tumour site. The drug taken up by the neighbouring healthy ceils and the fraction reaching to other organs of the body through blood may hamper the designated goal of site-specific drug delivery. Also, the observed high drug concentrations at the target-site following magnetic arterial chemoembolization does not guaranttee high drag concentrations at the intracellular sites. Unless released in a controlled manner, such a delivery device may rather lead to an accentuated drug toxicity. Based on the discussion detailed above, it is clear that the multiple tissue disposition of drugs delivered via a magnetic drug carriers has received limited attention. Only recently it has been shown that the drug exposure to a predef'med tail segment can be increased by using this carrier, as compared to the administration of equal dose of free drug (46). In view of the dose-dependent kinetics of the colloidal particles (98-103), it is likely that carrier dose may play a crucial role in deciding the extent to which the target site exposure can be increased. Lack of the optimisation of dose of a drug carrier, with regard to Table II Possible factors which may affect the retention ofmagnetieally responsive drug carriers at tumour site A. Characteristics of drug-cartier device 1. size 2. drug entrapment capacity 3. stability or release rate 4. magnetite content 5. dose relative to the capacity of target microvessels B. Characteristics of Magnet 1. strength 2. gradient 3. duration of application 4. specificity in field profile C. Characteristics of tumour site 1. vascularity 2. permeability 3. proximity with the applied magnetic field 4. proximity with the injection site
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the capacity of the target capillary bed, may result in carder overdosing. This may result in potentially unavoidable biodistribution of drug carder to non-target organs (104,105). Overall, several factors appears to affect the retention of carder at the target site, and hence the efficiency of drug targeting (see Table II). The size of the carrier and its magnetite content; strength, gradient and the time of application of the magnetic field; vascularity and permeability of tumour vasculature; these may all act towards deciding the extravasation of carrier, a condition prerequisite for achieving high concentration of drug at tumour level. Drug entrapment capacity and stability of the carrier may govern its amount required to achieve therapeutic concentrations in the target cells. These parameters may in turn require the validation of dose-dependent efficiency of carrier delivered drugs. Factors like magnetic field profile and proximity of tumour tissue to the applied field may influence the distribution of carrier in the target region and associated healthy tissues. Concludinu Remarks Magnetic drug carders have offered a new concept in drug targeting. Several studies on experimental animals carded out using these carriers have shown a moderate to high degree of success, in restraining the carrier away from the RES. However, this approach is applicable only for well defined tumours with abundant vascular supply, and hence its usefulness in treating metastatic neoplasms appears difficult. Nonetheless, these delivery systems may reduce the the chances of metastatic spread. Since this therapeutic approach warrants the application of a magnetic field, it is noteworthy that static magnetic fields of high gradient and strength do not cause any major side-effects on the biological system (106,107). However the use of non-homogeneous magnetic field may interact with the functions of erythrocytes (108) and alter the pattern of blood flow (109). In order to further appreciate the benefits and shortcomings of magnetic drug targeting in therapeutics, further work is needed to develop the methods which assist in the large scale production of delivery device with uniform physico-chemical characteristics. Indeed information is needed on drug localization ratios between tumour and normal sites within a given target volume, and between extracellular and intracellular compartments and target molecules in lesional loci, following the administration of a magnetic delivery device. Such an information would allow development of pharmacokinetic models which could be scaled up to human level (105,110). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
W. SADEE, In Topics in Pharmaceutical Scienc¢~, D.D. BREIMER and P.P. SPEISER, Eds. p. 123, Elsevier, North Holland, (1981). Y.N. LEE, K.K. CHAN, R.A. HARRIS and J.L. COHEN, Cancer, 45, 2231-2239 (1980). M.S. DIDOLKAR, P.M. KANTER, R.R. BAFFI, H.S. SCHWARTZ, R. LOPEZ and N. BAEZ, Ann. Surg. 187, 332-336 (1978). J.M. COLLINS, J. Clin. Oncol. 2, 498-504 (1984). U.G. ERIKSSON and T.N. TOZER, Acta Pharmacol. Toxicol. suppl. 59, 89 (1986). C. COLLIN, J. RIDGE, J. BADING, C. HANCOCK, P. CONTI, J. DALY and J. RAFT, Proc. Am. Ass. Cancer Res. 26, 352 (1985). H. BUCHWALD, T.B. GRAGE, P.P. VASSILOPOULOS, T.D. ROHDE, R.L. VARCO and P.J. BLACKSHEAR, Cancer, ~ 866-869 (1980). W. ENSMINGER, J. NIEDERHUBER, S. DAKHIL, J. THRALL and R. WHEELER, Cancer Treat. Rep. fi~, 393-400 (1981). J. LOKICH, A. BOTHE, T. ZIPOLI, R. GREEN, H. SONNEBORN, S. PAUL and D. PHILIPS, J. Clin. Oncol. 1, 24-28 (1983). G. COCCONI, G. BISAGNI, M. BACCHI, V.De. LISI, R. CANALETTI, A. CARPI, F. BUZZI and M.A. COLOZZA, Proc. Am. Ass. Cancer Res. 27, 190 (1986). D.F. CHIUTEN, K. LU, D. JEFFERIES, M.N. RABER, R.A., NEWMAN and J.A. NEIDHART, Proc. Am. Ass. Cancer Res. 27, 187 (1986). J. LOKICH, T. ZIPOLI and R. GREEN, Proc. Am. Ass. Cancer Res. 27, 178 (1986).
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Pergamon Press
MINIREVIEW
MAGNETICALLY CONTROLLED TARGETED MICRO-CARRIER SYSTEMS P.K. Gupta* and C.T. Hung§ Department of Pharmacy, University of Otago, Dunedin, New Zealand
Summary Magnetically controlled targeted drug delivery systems are aimed at concentrating drugs at a defined target site, with the aid of a magnetic field. This technique has been developed specifically for directing drugs away from the reticuloendothelial system (RES). Literature on this topic suggests that these delivery systems are capable of altering the distribution of chemotherapeutic agents in the body. Hence these delivery devices offer the possibility of improving the therapeutic efficacy of the associated drugs. This paper reviews the work done to date towards the development and evaluation of biodegradable and non-biodegradable magnetic targeted drug delivery systems and outlines their future prospects and limitations in cancer chemotherapy. Cytotoxic agents have extensively been employed in cancer chemotherapy since the early 1940's (1). However, the non-specificity of these drugs towards cancer and healthy cells of the body has curtailed their effective use in chemotherapy. Several modaiities have been tried to improve the therapeutic efficacy of these agents. These include multiple chemotherapy, regional drug delivery, long-term low-dose infusion, or short-term high-dose infusion of cytotoxic agents (2-12). However there is little quantitative evidence regarding the enhanced therapeutic efficacy of these dosing strategies over the conventional dosage regimens (13-15). The lack of specificity of these compounds has led scientists towards developing delivery systems which may improve their turnour : non-tumour tissue distribution in the body, so that the cumulative toxicities intrinsically associated with them are reduced. Targeted drug delivery systems are aimed at increasing the concentration and duration of action of drugs at the tumour site so that the survival of healthy cells is not considerably affected. The concept of magnetically controlled drug targeting probably emerged in 1970's when Zimmermann and Pilwat (16), and-Widder et al.(17,18) developed magnetic erythrocytes and magnetic albumin microspheres respectively, for the delivery of cytotoxic drugs. The unique feature of these delivery systems over other drug targeting techniques is their ability to minimise the carrier uptake by RES (19). Conceptually, the intended drug and a suitable magnetically active component (e.g., Fe304) are formulated into a pharmaceutically stable system. This is injected through the artery nourishing the tumour tissue in the presence of an external magnet which can generate sufficient field strength and gradient to confine the carrier at the target site. The magnetic field is directed exclusively onto the organ or tissue residing turnout cells for a sufficient period to allow the transfer of the carrier from the blood compartment to the extravascular target tissue. The drug released from the carrier in a controlled manner then exerts its pharmacological action at the cellular * To whom all correspondence should be addressed at : College of Pharmacy, University of Kentucky, Rose street, Lexington, Kentucky, USA. § Present address: Zenith Technology Corporation Limited, 156 Fredrick Street, P O Box 1777, Dunedin, New Zealand. 0024-3205/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc
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and/or sub-cellularlevelof the turnout tissuewithout criticallyaffectingthe survivalof normal tissue (19-21). This is an attractivemethod of drug localization,provided that the targeted tissue has abundant vascular supply and is accessible to magnetic fields. Investigationscarriedout using magnetic carrierhave demonstrated 10 to 100 fold increase in the therapeuticindex of the associateddrug (18,19,22,23).So far similarincrease in the therapeuticindex of chemotherapeutic agents have not been demonstrated by any other drug targetingdevice.
One of the major limitations of most drug delivery systems in cancer chemotherapy is their inability to extravasate the endothelium of the tumour tissue. Hence the included drug may not be available exclusively at the cellular and/or sub-cellular level (19,24). However it has been shown that magnetic drug targeting allows extravasation of carder and increases the probability of sub-cellular or third-order drug targeting (18,25-27). Slow release of an encapsulated cytotoxic agent prevents endothelial injury of the target tissue, which is usually observed following their administration as an infusion (28,29). The process of drug localization using magnetic delivery systems is based on the competition between forces exerted on the particles by blood compartment, and magnetic forces generated from the applied magnet. When the magnetic forces exceed the linear blood flow rates in arteries (10 cngsec) or capillaries (0.05 cm/sec), the microspheres (-1 ~tm) are retained at the target site and may be internalized by the endothelial cells of the target tissue (18,30,31). It has been suggested that at the arterio-capillary blood flow rate of 0.05 to 0.1 cm/sec, 20% w/w of magnetite is sufficient to achieve 100% retention of the magnetic carder using a 8000 Gauss magnet (30). Contrary to the drug targeting system discussed above, Kato (24) have developed large (200-300 gm) non-biodegradable magnetic microcapsules for drug targeting at the arterial level. Despite the fact that these particles do not extravasate the vascular endothelium of tumour tissue, and their ultimate fate in the body is not clearly understood, this carder has demonstrated promising results in experimental investigations (24,32). Table I shows a general classification of magnetically controlled targeted drug delivery systems, each of which is discussed in the subsequent part of this review. Table I Classification of magnetically controlled targeted drug delivery systems Class
Example(s)
A. BIODEGRADABLE 1. Particulate carders
Magnetic emulsions Magnetic starch microspheres Magnetic poly(alkyl cyanoacrylate) nanoparticles Magnetic albumin microspheres Magnetic erythrocytes Magnetic liposomes
2. Vesicular carders B. NON-BIODEGRADABLE 1. Particulate carriers
Magnetic ethylceUulose microcapsules
Magnetic emulsions All magnetic emulsions reported so far are of o/w type (33-36). The dispersed phase of these emulsions consists of a suspension of magnetite in oleic acid or ethyl-oleate,
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with caesin as the emulsifier. The effect of magnetic field strength (0, 2000, 4000 or 6000 Gauss) and field application time (10 or 60 rain) on the localization of magnetic ethyl-oleate emulsion, containing 14C-palmitic acid (36) or methyl-CCNU (35), to the lungs of rat has been investigated. Ten rain after administration of the emulsion through the tail vein, the radioactivity in lungs was 2 times higher in the presence of a magnet, compared to the control where no magnet was used. This level was 6 to 7 times higher than those analysed in liver, spleen, kidney and heart. However, 60 rain after administration, the radioactivity in lungs reduced appreciably, with concurrent increase in the radioactivity of liver and heart. It was suggested that the emulsion is washed from the lungs as a function of time (36). The data obtained at these two time points did not justify the use of higher magnetic fields for increasing the effectiveness of targeting. However, upon using the emulsion containing methyl-CCNU, remarkable increase in lung drug levels was noticed with an increase in magnetic field strengths (35). But no information was provided regarding the disposition of magnetic emulsion as a function of time. The possible reasons for the inefficiency of this delivery system in drug targeting include: (i) low magnetic susceptibility resulting into their poor magnetic drag and hence particle extravasation; (ii) short half-life of drug release due to large surface to volume ratio of the formulation; and (iii) marginal physical stability. Maunetic Starch Microsphere~ The use of magnetic starch microspheres in d~U¢l~ targeting was suggested by Mosbach and co-workers (37-40). Upon administration of 5I-albumin coupled magnetic starch microspheres (2-10 Ixm) through the marginal ear vein of rabbits, 4 to 8 fold increa in radioactivity in the ear, exposed to a 7000 Gauss for 10 rain, was demonstrated. Despite the fact that the animals were sacrificed immediately after removing from the magnetic field, 80% of the injected radioactivity was detected in the lungs. The large size of the microspheres accounted for this undesirable or negative targeting of the radioactivity. These studies also revealed in vivo de-iodination of the carrier. Potential use of this carrier in drug targeting requires further investigation. Magnetic nolv(alkvlcvanoacrvlate~ Nanooarticles The use of magnetic poly(isobutylcyanoacrylate) nanoparticles in circumventing the uptake of carrier by the liver has been suggested by Ibrahim et al. (41). In an in vitro experiment, these workers demonstrated that 28% w/w of magnetite in 0.22 gm nanoparticles is necessary for their effective targeting. Acute toxicity studies in mice showed that the LD50 of magnetic poly(isobutylcyanoacrylate)nanoparticles is equivalent to that of non-magnetic nanoparticles (i.e., 245 mg/kg). This study also confLrmed the non-toxicity of magnetite particles up to the doses of 1500 mg/kg. Upon intravenous administration of 3H-dactinomycin adsorbed magnetic nanoparticles, three fold increase in targeting to the left kidney of mice, exposed to a 8500 Gauss magnet for 10 rain, was observed in comparison to the right kidney which served as a control. Another experiment carried out with each kidney of mice exposecl to a 8500 Gauss magnet for 10 rain revealed three times higher radioactivity in kidneys and a third lower radioactivity in liver, compared to animals which were not exposed to magnets. However, in both experiments all the data were collected at only 10 rain after dosing and no information was provided on drug distribution to other tissues (41). It is emphasised that due to their toxic nature (42,43), poly(alkylcyanoacrylate) particles appear to possess low probability of regulatory approval. Like magnetic emulsions, these formulations also exhibit short half-life of drug release due to the presence of major drug load as an adsorbed fraction (44). Ma~etic Albumin Microsoheres Magnetic albumin micmspheres are micrometer size particles, routinely prepared by heat or chemical hardening of a w/o emulsion, consisting of drug, magnetite and albumin as dispersed droplets in a suitable oil (19,26,45-48). Studies investigating their physico-ehemical characteristics have revealed that the in vitro release of a cytotoxic drug
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from these particles can be controlled (48). However these studies have also demonstrated considerable batch to batch variation in the particle size and drug content of the magnetic microspheres (48). The within and between batch homogenity of these particles with respect to their drug and magnetite content is still unknown. The first in vivo model for verifying the drug targeting efficacy of magnetic albumin microspheres was proposed by Widder et al. (17,18). Rat's tail was used as a target organ and was demarcated into four equal regions (3.5 cm), called T1, T2, T3 and T4 respectively from the base of the tail. T1 was considered as the dose-administration site and T3 as the target-site. The magnetic drug carrier was infused intra-arteriaUy in a controlled manner so as to obtain a homogeneous capillary-level distribution of the particles within the magnetic targeting volume. In one experiment, the animals were sacrificed 30 rain after dosing with 0.5 mg of 125I-magnetic albumin microspheres in presence of a magnetic field applied for 30 rain (field strength 0, 4000, 6000 or 8000 Gauss). Although increase in magnetic field strength did result in a higher percentage of radioactivity at T3, the number of microspheres localized in liver, lungs and spleen did not decrease to any considerable extent. Shunting of blood vessels around the target region was suggested as the cause of microspheres pooling in the RES. Interestingly, the animals sacrificed at 24 hr after dosing also demonstrated high localization of radioactivity at T3. It was shown that the microspheres at T3 pass from the vascular compartment into the interstitial tissue, preventing their delocalization after removing the magnet (18). To verify the effect of magnetic field application time on the in vivo distribution of low-dose adriamycin, Senyei et al. (22). administered 0.05 mg/kg of 125I-magnetic albumin microspheres in rats, with the T3 region exposed to a field of 8000 G for 5 to 60 rain. The animals were sacrificed immediately after removing the field and various tissues analysed for 125I-counts. Administration of equal dose of microspheres in the absence of magnetic field served as a control. Regardless of the field application time, this study demonstrated 7 to 10 times higher radioactivity at T3, compared to those at liver, lung or spleen. The control animals demonstrated no radioactivity in tail segments. In addition these animals indicated twice the radioactivity in liver, lung and spleen than the treatment groups (22). Since the animals were sacrificed immediately after removing the magnetic field, this study could not identify the effect of a field application time on the actual disposition of the magnetic albumin microspheres, several hours after removing the field. However this study revealed that at a 60 rain post-administration point, a 0.05 mg/kg microsphere dose produced target-site adriamycin concentrations twice as high as that resulted from 5 mg/kg intravenous solution dose (22). The efficacy of magnetic albumin microspheres in the tumour bearing animals have also been investigated (49,50). Rats transplanted with Yoshida sarcoma at T3 were administered with a single dose of 0.5 mg/kg adriamycin in the form of magnetic albumin microspheres, in presence of 5500 Gauss magnetic field applied for 30 min post-dosing. Results were compared with six controlled experiments: (i) untreated animals; (ii) 5 mg/kg free adriamycin given intravenously; (iii) 5 mg/kg free adriamycin given intra-arterially; (iv) 0.5 mg/kg free adriamycin given intra-arterially; (v) equivalent dose of non-drug bearing magnetic albumin microspheres administered in presence of a magnet; and (vi) 0.5 mg/kg adriamycin as magnetic albumin microspheres given in absence of magnet. Whereas 80 to 100% of the animals in the controlled groups died between day 13 and 16 after tumour inoculation, 100% of the animals survived for up to 30 days (treatment time) in the group treated with 0.5 mg/kg adriamyein via magnetic carder in presence of magnet. In addition, 75% of the animals (9/12) in this group exhibited complete remission of tumour and another 17% (2/12) showed def'mite tumour regression (50). One animal showed no change in tumour size during the treatment time (30 days). These results were verified in another study (51), where 0.5 or 2.5 mg/kg adriamycin was administered using the magnetic carrier, in presence of a 5500 Gauss magnet. This study revealed no significant difference between the two adriamycin doses, when given as magnetic carrier for the treatment of tumour bearing animals (51).
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In a separate study where drug-free magnetic albumin microspheres were targeted to Yoshida sarcoma in rat's tail, Widder et al. (25) demonstrated extravasation of microspheres as early as 10 rain after their administration. The microspheres were shown to be endocytosed by tumour. However the actual uptake of microspheres occurred in a very small fraction of ceils. Using adriamycin-associated magnetic albumin microspheres in healthy animals, second-order drug targeting has been confLrrned in our laboratory (26). Based on these results it is evident that this therapy could be beneficial for drugs discarded in cancer chemotherapy due to their poor cell uptake. An extensive study on the effect of magnet type on the targeting efficiency of magnetic albumin microspheres to rat's tail, and their localization to other organs, has been conducted by Ovadia et al. (52). This study proved that a custom-made bar magnet (field strength 4200 Gauss; gradient at pole face: 2700 Gauss/cm) was as effective as a horse-shoe magnet (strength 7000 Gauss; gradient 4000 Gauss/era), in localizing the magnetic carrier at the tail target site (T3). These results led to the conclusion that the magnetic field gradient, rather than strength, is important in the retention of microspheres. It also was suggested that the magnetic responsiveness of these particles may achieve saturation at field strengths greater than 4000 Gauss (52). In addition, this study showed that the complexity of target site vasculature, and its depth and permeability, may restrict the application of magnetic albumin microspheres in localized drug therapy. It was concluded that to achieve maximum targeting efficiency, the magnetic carrier should be administered closest to the applied magnetic field (52). In a study comparing the size of the magnetic carrier (1 versus 3 ~tm) on its localization to lungs, in presence of a 3000 Gauss magnet, use of 3 ~tm microspheres demonstrated superior results than the smaller carrier (53). In a subsequent study, these workers (43) monitored the localization of 125I-magnetic albumin microspheres to the lungs and the left kidney of mice and rats, respectively. In both experiments, 3000 Gauss magnets were applied for 10 or 60 rain post-dosing, and the radioactivity at the target-site was monitored at 10 as well as 60 rain after dosing. In mice, when the magnets were used throughout the experiment, the lung radioactivity increased from 17% of the injected dose at 10 rain to 28.2% at 60 min. However when the field was applied for only 10 min, the radioactivity at 60 rain decreased substantially as compared to that at 10 rain (54). In rat kidney, when the field was applied throughout the experiment, no increase in the localization of the radioactivity was noticed at 60 rain, as compared to that at 10 min. But when the field was applied only for 10 rain, the radioactivity decreased from 56.4% of the injected dose at 10 rain to 25.5% at 60 min, with simultaneous increase in liver radioactivity. The reduction in targeting efficiency as a function of time was suggested as a result of the redistribution of microspheres in the absence of a magnetic field. It was therefore proposed that the magnetic field application time should be prolonged to facilitate the retention of microspheres at the target site (54). The larger microspheres (e.g. 3 Ixm) localize upstream in the vessels where they encounter greater resistance for the extravasation due to thicker epithelium of the vessel wall. It is also emphasised that due to-the application of a magnetic field, the microspheres tend to aggregate and cluster into longitudinally aligned short cylindrical chains (31), and this process is rarely affected by the blood flow. Due to this phenomenon, the functional diameter of microspheres in magnetic field is significantly greater than the actual diameter observed under an electron microscope. In a study carried out on rats with AH 7974 lung metastases (55), use of adriamycin-associated magnetic albumin microspheres, in the presence of 6000 Gauss magnet applied for 10 rain, demonstrated increased survival of animals compared to the untreated animals, or animals treated with the free drug. A study by Bartlett et al. (56) in the canine osteosarcoma of a limb of dogs demonstrated little difference between the routes of administration and infusion pattern of the suspension of a radioactive magnetic albumin microspheres, towards the retention of the particles in the tumour tissue, 15 min after the removal of an electromagnet from the target region. However this study revealed that the localization of microspheres occurs
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only in the weU-perfused and soft tissue regions of the tumour. Insignificant radioactivity was traced in the necrotic or the osseous portions of the tumour. Using magnetic albumin microspheres containing 10% w/w of vindesine sulphate, Morris et al. (57) have demonstrated 85% total remission and almost 95% regression in Yoshida sarcoma implanted in rat tail. This study also confirmed no significant difference between the 0.5 and 2.5 mg/kg doses of vindesine for the eradication of sarcoma, when delivered via the magnetic carrier. The control animals receiving either free or non-rnagnetieaUy targeted drug-loaded microspheres displayed widespread metastases and ultimate death (57). In an effort to modulate localized inflammation, Ranney (27) has recently demonstrated successful targeted delivery of N-formyl methionylleueyl phenylalanine to rat lungs via magnetic albumin microspheres, in presence of a 5500 Gauss thoracic magnet. The targeted mierospheres were observed in the extravascular compartment as early as 5 rain post-administration. The targeted bio-modulator induced local accumulation of neutrophils and acute tissue injury due to chemotaxis. A study investigating the multiple tissue disposition of adriamycin, intra-arterially targeted to rat tail via the magnetic albumin microspheres, in presence of a 8000 Gauss magnet applied for 30 rain, revealed 64% higher exposure of the target-site compared to the equivalent dose administered as a free drug (see Fig. 1) (46). Although most other tissues, including heart, demonstrated upto 75% reduction in their exposure towards the magnetically targeted adriamycin, liver uptake of the carrier could not be prevented (46). 20
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Time (Hrs) Fig. 1 Log-linear plots of tissue concentration versus time for the tail target-site (A) and liver (B), following the administration of 2.0 mg/kg adriamycin HC1 as a solution (r~) or via the magnetic albumin microspheres, in presence of a 8000 Gauss electromagnet applied for 30 min post-dosing (e). Each point represent mean of three rats. (Adapted in part from reference 46). .Magnetic Ervthrocvtes The clinical use of cell transfusions has led to the use of erythrocytes in drug delivery and targeting. As well as being biodegradable even after chemical treatment, use of a patient's own erythrocytes precludes the possibility of immunological responses often encountered with other exogenous drug carriers (58-62). Zimmermann and Pilwat (16) proposed the use of magnetic erythrocytes in site-specific delivery of drugs. According to these workers, magnetic particles (10-20 nm) can easily be incorporated in erythrocyte ghosts at an electric field intensity of 15 KV/cm and a pulse length of 50 I.tsec, in isotonic solutions of sucrose, glucose or mannitol (63). It has been claimed that the targeting
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Magnetically COntolled Drug Delivery
efficiency of these carriers can be as high as 70% and their release profiles can be controlled by adjusting the stability of cell membranes or by electric lysis of cell in vivo (63-65). However the premature release of drug due to inadequate membrane stability and poor penetration of this carrier through tumour endothelium (19,66) have curtailed its pharmaceutical scale-up and further experimental evaluation in drug targeting. Magnetic Lipo~omes Liposomes is a particular class of drug carrier which has been studied more extensively than any other existing targeting drug delivery system. These consist of one or more concentric bilayer structure in a fashion that the lipid layers alternate with the aqueous layers (66-71). They exist in a size range of a few nanometers to several microns. A unique feature associated with these carders is their ability to incorporate both water soluble and oil soluble therapeutic agents in the aqueous and the lipid layers, respectively. It has been claimed that judicious choice of lipid composition, size, surface charge and surface groups may control the in vivo fate of drugs incorporated in liposomes (67,72,73). However the entrapment capacity of most liposomal preparations is < 5% w/w, and this carrier is rapidly cleared by RES. Increased popularity and successful experimental application of magnetic drug targeting has led to the development of magnetically responsive liposomes. K.iwada et al. (74) has demonstrated meagre but significant increase in the localisation of 3H-inulin delivered via this carrier in Yoshida sarcoma implanted in foot pad of rats. It was concluded that increase in magnetic field strength may increase the turnout versus non-tumour tissue distribution of drugs administered via this carrier. In view of the poor stability of liposomes, it is rather difficuk to predict its future prospects in drug targeting. Magnetic Ethvlcellulose Microcansules Transcatheter vessel occlusion for the management of hemorrhage, spenomegaly and radiotherapy of carcinomas has been practiced since last two decades (75-78). This technique is still popular for inducing temporary hypoxia and altering the distribution of chemotherapeutic agents (79-84). The concept of embolization, followed by chemotherapy of carcinomas has been successfully promoted by Kato et al. (85-88) and the technique has been named as arterial chemoembolization. This method involves infusion of drug-loaded microcapsules after arterial catheterization of the tumour tissue (24,89,90). Due to their large size (200-300 ~tm), these particles along with the drug are localised near the target site immediately after their administration. This leads to ischemia and anoxia of tumour tissue. In addition, sustained release of drug in high concentration increases its contact time with tumour cells (24). This approach is said to preclude the necessity of carrier extravasation. It is believed that intrinsic permeability of tumour tissue or increase in tissue permeability due to anoxia, assists rapid cellular action of the associated drug (24). Despite its clinical success, arterial chemoembolization has been found to be non-practicle for drug targeting in carcinomas with complicated vascular supply. In addition, existence of arterio-venous fistula in tumours may reduce the probability of carrier occlusion at the tumour site (24). To overcome these problems, ethylcellulose microcapsules with 16% w/w zinc ferrite and 50% w/w mitomycin C, in presence of a suitable magnet, have been tried. This technique is known as magnetic arterial chemoembolization (32,91-94). In an experiment utilizing rabbits with bladder tumour, use of drug-free magnetic microcapsules in presence of magnet, or drug-loaded microcapsules in absence of a magnet, displayed no response towards tumour eradication. However, use of drug-loaded magnetic microcapsules in presence of a magnet (3500 Gauss/era) demonstrated successful necrosis of the tumour (32). In subsequent experiments on rabbits with transplanted VX2 carcinoma in hind limb, intra-arterial administration of mitomycin C via magnetic microcapsules, in presence of a 1250 Gauss magnet applied for 30 rnin, displayed marked reduction in tumour size over a period of 40 days. The animals receiving free drug or no drug showed 12 fold increase in tumour size during the same period, as compared to the treatment group (95). However some animals died in the treatment group, due to infection following the necrosis of the foot.
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The feasibility of magnetic microcapsules in intra-vesicular administration of drugs has also been investigated (96,97). In one study, it was shown that intra-vesicular administration of free mitomycin C (control i) or placebo microcapsules plus free drug (control ii) resulted in no detectable drug concentrations in the urinary bladder. However administration of mitomycin C loaded magnetic microcapsules, in presence of a 2400 Gauss magnet fixed onto the external wall of urinary bladder, resulted in significant amount of drug levels in the target tissue. Nonetheless, this group also demonstrated 4 to 6 times higher drug levels in blood compared to the controlled groups (96,97). Despite several encouraging reports, the concept of magnetic arterial embolization has failed to receive the anticipated impact mainly due to the large size and non-biodegradability of the carrier. Inability of this carder to extravasate from the embolizing vessel precludes the possibility of effective chemotherapy, as demonstrated with magnetic albumin rnicrospheres at the intracellular level (18,26,27). Long term occlusion of tumour tissue vasculature may favour the formation of thrombus, and affect the activity of associated healthy tissues. Occlusion of a blood vessel by the carder means that all the released drug may not necessarily be available for cytotoxic action at the tumour site. The drug taken up by the neighbouring healthy ceils and the fraction reaching to other organs of the body through blood may hamper the designated goal of site-specific drug delivery. Also, the observed high drug concentrations at the target-site following magnetic arterial chemoembolization does not guaranttee high drag concentrations at the intracellular sites. Unless released in a controlled manner, such a delivery device may rather lead to an accentuated drug toxicity. Based on the discussion detailed above, it is clear that the multiple tissue disposition of drugs delivered via a magnetic drug carriers has received limited attention. Only recently it has been shown that the drug exposure to a predef'med tail segment can be increased by using this carrier, as compared to the administration of equal dose of free drug (46). In view of the dose-dependent kinetics of the colloidal particles (98-103), it is likely that carrier dose may play a crucial role in deciding the extent to which the target site exposure can be increased. Lack of the optimisation of dose of a drug carrier, with regard to Table II Possible factors which may affect the retention ofmagnetieally responsive drug carriers at tumour site A. Characteristics of drug-cartier device 1. size 2. drug entrapment capacity 3. stability or release rate 4. magnetite content 5. dose relative to the capacity of target microvessels B. Characteristics of Magnet 1. strength 2. gradient 3. duration of application 4. specificity in field profile C. Characteristics of tumour site 1. vascularity 2. permeability 3. proximity with the applied magnetic field 4. proximity with the injection site
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Magnetically Controlled Drug Delivery
the capacity of the target capillary bed, may result in carder overdosing. This may result in potentially unavoidable biodistribution of drug carder to non-target organs (104,105). Overall, several factors appears to affect the retention of carder at the target site, and hence the efficiency of drug targeting (see Table II). The size of the carrier and its magnetite content; strength, gradient and the time of application of the magnetic field; vascularity and permeability of tumour vasculature; these may all act towards deciding the extravasation of carrier, a condition prerequisite for achieving high concentration of drug at tumour level. Drug entrapment capacity and stability of the carrier may govern its amount required to achieve therapeutic concentrations in the target cells. These parameters may in turn require the validation of dose-dependent efficiency of carrier delivered drugs. Factors like magnetic field profile and proximity of tumour tissue to the applied field may influence the distribution of carrier in the target region and associated healthy tissues. Concludinu Remarks Magnetic drug carders have offered a new concept in drug targeting. Several studies on experimental animals carded out using these carriers have shown a moderate to high degree of success, in restraining the carrier away from the RES. However, this approach is applicable only for well defined tumours with abundant vascular supply, and hence its usefulness in treating metastatic neoplasms appears difficult. Nonetheless, these delivery systems may reduce the the chances of metastatic spread. Since this therapeutic approach warrants the application of a magnetic field, it is noteworthy that static magnetic fields of high gradient and strength do not cause any major side-effects on the biological system (106,107). However the use of non-homogeneous magnetic field may interact with the functions of erythrocytes (108) and alter the pattern of blood flow (109). In order to further appreciate the benefits and shortcomings of magnetic drug targeting in therapeutics, further work is needed to develop the methods which assist in the large scale production of delivery device with uniform physico-chemical characteristics. Indeed information is needed on drug localization ratios between tumour and normal sites within a given target volume, and between extracellular and intracellular compartments and target molecules in lesional loci, following the administration of a magnetic delivery device. Such an information would allow development of pharmacokinetic models which could be scaled up to human level (105,110). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
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Magnetically Controlled Drug Delivery
Vol. 44, No. 3, 1989
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