In vivo and in vitro release of macromolecules from polymeric drug delivery systems

B'reshith

The Hebrew word "b'reshith" (בְּרֵאשִׁית) means "in the beginning". It is the first word and title of the Book of Genesis, and it describes a process of creation. The four authors were present at the beginning of Langer labs, and the purpose of this essay is to convey the scientific and technological zeitgeist that existed in the late 1970s and early 1980s, when Bob Langer began his exceptionally creative work. While Langer labs has branched into many other areas, Bob's unique ability to recognize important problems and entice people to look beyond their own disciplines to solve them was evident from the start. We focus on the two areas of most interest to Bob at the time, namely controlled release of macromolecules from polymers, and removal of heparin in order to prevent uncontrolled bleeding during surgery.

Current status of local therapy in malignant gliomas--a clinical review of three selected approaches

Malignant gliomas are the most frequently occurring, devastating primary brain tumors, and are coupled with a poor survival rate. Despite the fact that complete neurosurgical resection of these tumors is impossible in consideration of their infiltrating nature, surgical resection followed by adjuvant therapeutics, including radiation therapy and chemotherapy, is still the current standard therapy. Systemic chemotherapy is restricted by the blood-brain barrier, while methods of local delivery, such as with drug-impregnated wafers, convection-enhanced drug delivery, or direct perilesional injections, present attractive ways to circumvent these barriers. These methods are promising ways for direct delivery of either standard chemotherapeutic or new anti-cancer agents. Several clinical trials showed controversial results relating to the influence of a local delivery of chemotherapy on the survival of patients with both recurrent and newly diagnosed malignant gliomas. Our article will review the development of the drug-impregnated release, as well as convection-enhanced delivery and the direct injection into brain tissue, which has been used predominantly in gene-therapy trials. Further, it will focus on the use of convection-enhanced delivery in the treatment of patients with malignant gliomas, placing special emphasis on potential shortcomings in past clinical trials. Although there is a strong need for new or additional therapeutic strategies in the treatment of malignant gliomas, and although local delivery of chemotherapy in those tumors might be a powerful tool, local therapy is used only sporadically nowadays. Thus, we have to learn from our mistakes in the past and we strongly encourage future developments in this field.

Polifeprosan 20, 3.85% carmustine slow-release wafer in malignant glioma: evidence for role in era of standard adjuvant temozolomide

The Polifeprosan 20 with carmustine (BCNU, bis-chloroethylnitrosourea, Gliadel^®) polymer implant wafer is a biodegradable compound containing 3.85% carmustine which slowly degrades to release carmustine and protects it from exposure to water with resultant hydrolysis until the time of release. The carmustine implant wafer was demonstrated to improve survival in blinded placebo-controlled trials in selected patients with newly diagnosed or recurrent malignant glioma, with little increased risk of adverse events. Based on these trials and other supporting data, US and European regulatory authorities granted approval for its use in recurrent and newly diagnosed malignant glioma, and it remains the only approved local treatment. The preclinical and clinical data suggest that it is optimally utilized primarily in the proportion of patients who may have total or near total removal of gross tumor. The aim of this work was to review the evidence for the use of carmustine implants in the management of malignant astrocytoma (World Health Organization grades III and IV), including newly diagnosed and recurrent disease, especially in the setting of a standard of care that has changed since the randomized trials were completed. Therapy has evolved such that patients now generally receive temozolomide chemotherapy during and after radiotherapy treatment. For patients undergoing repeat resection for malignant glioma, a randomized, blinded, placebo-controlled trial demonstrated a median survival for 110 patients who received carmustine polymers of 31 weeks compared with 23 weeks for 122 patients who only received placebo polymers. The benefit achieved statistical significance only on analysis adjusting for prognostic factors rather than for the randomized groups as a whole (hazard ratio = 0.67, P = 0.006). A blinded, placebo-controlled trial has also been performed for carmustine implant placement in newly diagnosed patients prior to standard radiotherapy. Median survival was improved from 11.6 to 13.9 months (P = 0.03), with a 29% reduction in the risk of death. When patients with glioblastoma multiforme alone were analyzed, the median survival improved from 11.4 to 13.5 months, but this improvement was not statistically significant. When a Cox’s proportional hazard model was utilized to account for other potential prognostic factors, there was a significant 31% reduction in the risk of death (P = 0.04) in this subgroup. Data from other small reports support these results and confirm that the incidence of adverse events does not appear to be increased meaningfully. Given the poor prognosis without possibility of cure, these benefits from a treatment with a favorable safety profile were considered meaningful. There is randomized evidence to support the use of carmustine wafers placed during resection of recurrent disease. Therefore, although there is limited specific evidence, this treatment is likely to be efficacious in an environment when nearly all patients receive temozolomide as part of initial management. Given that half of the patients in the randomized trial assessing the value of carmustine implants in recurrent disease had received prior chemotherapy, it is likely that this remains a valuable treatment at the time of repeat resection, even after temozolomide. There are data from multiple reports to support safety. Although there is randomized evidence to support the use of this therapy in newly diagnosed patients who will receive radiotherapy alone, it is now standard to administer both adjuvant temozolomide and radiotherapy. There are survival outcome reports for small cohorts of patients receiving temozolomide with radiotherapy, but this information is not sufficient to support firm recommendations. Based on the rationale and evidence of safety, this approach appears to be a reasonable option as more information is acquired. Available data support the safety of using carmustine wafers in this circumstance, although special attention to surgical guidelines for implanting the wafers is warranted.

A novel approach for the preparation of highly loaded polymeric controlled release dosage forms of diltiazem HCl and diclofenac sodium

In this investigation, modified-release dosage forms of diltiazem HCl (DT) and diclofenac sodium (DS) were prepared. The development work comprised two main parts: (a) loading the drug into ethylene vinyl acetate (EVA) polymer, and (b) generation of a non-uniform concentration distribution of the drug within the polymer matrix. Phase separation technique was successfully used to load DT and DS into the polymer at significantly high levels, up to 81 and 76%, respectively. Size diameter of the resultant microspheres was between 1.6 and 2.0mm. Controlled-extraction of loaded microspheres and high vacuum freeze-drying were used to generate the non-uniform concentration distribution and to immobilize the new drug distribution within the matrix. Parameters controlling the different processes were investigated, and hence optimal processing conditions were used to prepare the dosage forms. Rates of drug release from the two dosage forms in water and in media having different pH were found to be constant for an appreciable length of time (>8h) followed by a slow decline; a characteristic of a non-Fickian diffusion process. Scanning electron microscopy studies suggested that the resultant release behavior was the outcome of the combined effects of the non-uniform distribution of the drug in the matrix and the apparent changes in the pores and surface characteristics of the microspheres. Comparison of release rate-time plots of dissolution data of marketed products with the newly developed dosage forms indicated the ability of the latter to sustain more zero order release.

Therapeutic applications of implantable drug delivery systems

In the past, drugs were frequently administered orally, as liquids or in powder forms. To avoid problems incurred through the utilization of the oral route of drug administration, new dosage forms containing the drug(s) were introduced. As time progressed, there was a need for delivery systems that could maintain a steady release of drug to the specific site of action. Therefore, drug delivery systems were developed to optimize the therapeutic properties of drug products and render them more safe, effective, and reliable. Implantable drug delivery systems (IDDS) are an example of such systems available for therapeutic use. The application of currently available implantable drug delivery systems is the main focus of this review. IDDS can be classified into three major categories: biodegradable or nonbiodegradable implants, implantable pump systems, and the newest atypical class of implants. Biodegradable and nonbiodegradable implants are available as monolithic systems or reservoir systems. The release kinetics of drugs from such systems depend on both the solubility and diffusion coefficient of the drug in the polymer, the drug load, as well as the in vivo degradation rate of the polymer, especially, in the case of the biodegradable systems. Controlled release of drug from the implantable pump is generally achieved utilizing the microtechnology of electronic systems and remote-controlled flow rate manipulation through the maintenance of a constant pressure difference. The third atypical class includes those which have been recently developed such as ceramic hydroxyapatite antibiotic systems used in the treatment of bone infections, intraocular implants for the treatment of glaucoma, and transurethral implants utilized in the treatment of impotence. The major advantages of these systems include targeted local delivery of drugs at a constant rate, less drug required to treat the disease state, minimization of possible side effects, and enhanced efficacy of treatment. Also, these forms of delivery systems are capable of protecting drugs which are unstable in vivo and that would normally require a frequent dosing intervals. Due to the development of such sustained release formulations, it is now possible to administer unstable drugs once a week to once a year that in the past required frequent daily dosing. Preliminary studies using these systems have shown superior effectiveness over conventional methods of treatment. However, one limitation of these newly developed drug delivery systems is the fact that their cost-to-benefit ratio (cost/benefit) is too high which restricts their use over conventional dosage forms. Hopefully, in the future, new implantable systems can be developed at a lower cost, thereby minimizing the cost-to-benefit ratio and therefore, be used extensively in standard therapeutic practice. Some of the most recently discovered implants are in the early developmental stages and more rigorous clinical testing is required prior to their use in standard practice.

Prevention of acute inducible atrial flutter in dogs by using an ibutilide-polymer-coated pacing electrode

Atrial arrhythmias (atrial fibrillation or atrial flutter) after coronary artery bypass graft surgery are difficult to prevent or treat and often result in significant morbidity. Prior experimental studies by our group showed improved therapeutic efficacy for antiarrhythmic drugs delivered via controlled-release polymeric matrices implanted on the epicardial surface. These experiments were conducted to test the hypothesis that direct atrial epicardial administration of ibutilide from a controlled-release system (compared with intravenous administration) can reduce the inducibility of atrial flutter in the acute postoperative atrial myocardium. Polymeric sustained-release preparations were formulated by solvent casting of an ibutilide and polyurethane (Pellathane) solution in tetrahydrofurane. Multilayer solvent-casted coatings on pacing electrode wires were carried out to fabricate a sustained-release electrode system. In animal model studies, each dog underwent a thoracotomy, followed by a right atriotomy that was repaired. Induction of atrial flutter was attempted by burst pacing with the bipolar pacing catheter. Sinus rhythm was restored with overdrive pacing. After determining the induction rate (percentage) of atrial flutter in the baseline state, a stainless-steel wire coated with the drug-delivery system, 10% ibutilide/90% polyurethane (n = 7), or without drug (polyurethane coating without ibutilide, n = 5; control) was sewn onto the right atrium. Systemic intravenous administration of ibutilide (1.2 microg/kg/h) also was carried out in a separate group of animals after atriotomy (n = 5). For ibutilide (at an estimated dose of 1.2 microg/kg/h), the atrial-flutter results were 41.85 +/- 2.21% induction for baseline compared with 12.42 +/- 5.26% (p = 0.02) after the ibutilide wire implant. In the control dogs, atrial flutter was induced 29.4 +/- 4.7% at baseline and 25.2 +/- 5.1% after implantation of the control wire (p = 0.4). Ibutilide coronary venous serum concentrations at the end of the ibutilide-polyurethane electrode experiments were 2.25 +/- 0.2 ng/ml (mean +/- SEM) versus systemic levels that were below the limits of detection. Systemic intravenous ibutilide infusions had no effect on the inducibility of atrial flutter. In conclusion, an epicardial implantable electrode coating with an ibutilide controlled drug-release system significantly reduced the inducibility of atrial flutter in an experimental atriotomy model. These results suggest that atrial arrhythmias occurring after coronary bypass surgery may be prevented by local atrial administration of ibutilide from a controlled-release pacing electrode.

Drug clearance and arterial uptake after local perivascular delivery to the rat carotid artery

OBJECTIVES

We attempted to characterize how drug released into the perivascular space enters the arterial wall and how it is cleared from the local environment.

BACKGROUND

Drug released into the perivascular space can enter the artery either from the adventitial aspect or from the lumen after absorption by the extraarterial capillaries and mixing within the systemic circulation. Some investigators suggest that this latter mechanism dominates, and they question whether local drug release is synonymous with local deposition.

METHODS

We investigated both the pathways by which adventitially released drug is cleared from the perivascular space and those by which drug enters the blood vessel wall. Inulin was used to follow drug release from implanted devices and subsequent entry to the circulation, because of its first-pass urinary excretion. Heparin was used to follow arterial deposition because of its vasoactivity and tissue-binding properties. The different potential pathways of drug entry and egress were systematically removed and the effects on metabolism and deposition determined.

RESULTS

Ligature occlusion of the artery did not decrease inulin excretion or heparin deposition. Extravascular wraps designed to shield the device from extramural capillaries reduced inulin excretion rates 10-fold but did not alter heparin deposition into the vessel wall. The deposition of drug after perivascular delivery was 500 times higher than after intraperitoneal administration.

CONCLUSIONS

Although almost all the drug released into the perivascular space is cleared through the extravascular capillaries, virtually all the deposited drug diffuses directly from the perivascular space, and little arrives from the endovascular aspect. These data support the view that local drug release leads directly to increased local drug concentration.

Quantification of insulin release from implantable polymer-based delivery systems and augmentation of therapeutic effect with simultaneous release of somatostatin

Insulin injections control diabetes mellitus but do not reproduce physiologic regulation. Polymer-based controlled-release technology has enabled us to demonstrate: that the controlled release of insulin from polymer matrices can indeed be used to control diabetes mellitus but does so at the expense of hyperinsulinemia and hypoglycemia; and that somatostatin can be delivered in similar fashion, so as to provide glucose homeostasis in a more physiologic range, at lower insulin levels and at somatostatin doses below those used in intermittent infusion studies; and, that microgram quantities of a drug can be delivered successfully in vivo with intact biological function and in a manner that can be monitored continuously. In the present study the simultaneous polymer-matrix-controlled release of insulin with somatostatin extended glycemic control in diabetic rats. Eleven rats received subcutaneous polymer matrix implants containing insulin alone and 11 rats received implants containing insulin and somatostatin. Plasma and urinary glucose control were improved in both groups. Glucose concentrations in the insulin alone group remained depressed for 5 days until insulin release from the matrices declined below 11.6 units/kg/day. When somatostatin was delivered at 0.75-1.1 micrograms/kg/day together with insulin, plasma glucose control persisted for 12 days until insulin release decreased below 3.6 units/kg/day. It is our hope that further experiments regarding the potential role of both controlled-release devices and somatostatin will be performed to provide continuing therapeutic alternatives to the insulin-dependent diabetic. This is also the first in vivo demonstration of the simultaneous release of two biologically active peptide hormones from polymer matrices. The use of the polymer matrix systems may not only have profound effects on the ambulatory care of diabetes but might also permit the investigation of the synergistic effects of other families of compounds.

Characterization of glucose-mediated insulin release from implantable polymers

We characterized a glucose-sensitive, controlled-release insulin delivery system. Insulin release rates increased when glucose was perfused in the release media surrounding the matrix. The system was composed of solid, particulate insulin, incorporated into an ethylene-vinyl acetate copolymer (EVAc) matrix. Feedback control was mediated by the glucose oxidase enzyme immobilized to Sepharose beads, which were incorporated along with insulin into the EVAc matrix. When glucose in solution entered the insulin delivery system, gluconic acid was produced, causing a drop in the microenvironmental pH of the matrix. This fall in pH resulted in a rise in insulin solubility and consequently a rise in the insulin release rate from the matrix. Insulin concentrations increased in vitro and in vivo in response to glucose infusion. The increased insulin release was shown to consist of a finite pulse of insulin that required an optimal recovery period of 1 h to achieve a maximal repeated response to a glucose stimulus. Repeated pulses were demonstrated over a 4 h period. An optimum enzyme ratio was also determined.

Polymeric controlled-release amsacrine chemotherapy in an experimental glioma model

The purpose of this study was to fabricate and investigate amsacrine containing polymeric rods for use in interstitial chemotherapy of malignant glioma. Ethylene vinyl acetate copolymer (EVAc) rods containing 40% amsacrine (AMSA) were fabricated successfully with an extrusion method. In vitro kinetic studies revealed a high level of reproducibility of the production process. The release of AMSA showed a biphasic pattern consistent with a matrix-type controlled-release system with an initial more rapid release rate followed by a slower and more linear release phase. Release of AMSA was observed for over 6 months and the rods continue to release in a stable fashion. In vitro studies using rat glioma (RG2) in cell culture showed that cells treated with AMSA released from the rods were killed in a dose dependent manner indicating that AMSA incorporated into the polymer remained biologically active. In vivo studies of rats with single AMSA rods implanted five days after RG2 tumour implantation revealed histological evidence of an anti-tumour effect as well as an increased survival (p < 0.0003). The mean survival of the amsacrine treated rats was 78 days with 50% still remaining alive > 5 months after implantation. All control animals developed tumours and died within 15-19 days after tumour implantation (mean = 17 days). Amsacrine implanted animals showed no significant histological or clinical evidence of toxicity. We conclude that amsacrine containing EVAc rods can be safely and efficaciously use against the RG2 experimental glioma in a rat model and warrant further investigation.

In vitro and in vivo studies of subcutaneous hydromorphone implants designed for the treatment of cancer pain

Unrelieved cancer pain remains a significant problem worldwide. Patients receive inadequate analgesia for a variety of complex and multifactorial reasons. Limited availability of opioids secondary to concerns about potential diversion of these medications for illicit use and poor compliance with oral regimens are significant factors in many countries. This study was designed to develop and test an implantable opioid delivery device capable of releasing a potent opioid subcutaneously at a continuous rate for 4 weeks. A low temperature solvent casting technique was used to formulate ethylene vinyl acetate (EVA) copolymer disks containing 50% hydromorphone by weight. The release characteristics of disks of different height and diameter, coated and uncoated, and with and without a central uncoated channel were studied. The effect of temperature and pH were also evaluated. In vitro assessments were conducted in phosphate buffer using UV spectrophotometry. In vivo studies employed New Zealand White Rabbits and a radioimmunoassay. Plasma levels following hydromorphone delivery by polymer, osmotic pump, and intravenous administration were compared. In vitro, uncoated EVA polymer disks measuring 1.05 cm in diameter and 0.27 cm in height released an initial large burst of hydromorphone. Coating the disks with 100-200 microM of poly(methyl-methacrylate) prevented drug egress from the polymer. A central uncoated channel measuring 1.25 mm in diameter in an otherwise coated polymer virtually eliminated the initial burst of drug release and provided near zero-order hydromorphone release at an average rate of 164 micrograms per hour for 4 weeks. Doubling the height of the polymer approximately doubled the release rate while doubling the diameter of the polymer extended the duration of drug release to over 8 weeks. In rabbits, stable plasma hydromorphone concentrations (23-37 ng/ml) were sustained for 4 weeks following implantation of 2 polymers with an uncoated central channel. No initial burst of hydromorphone release was noted. Increasing the number of polymers produced sustained and predictable increases in plasma hydromorphone concentrations. Plasma levels were similar with subcutaneous hydromorphone delivered by polymer and osmotic pump and much less variable than with intravenous bolus hydromorphone. A uniquely configured implantable drug delivery device has been developed using materials which are approved for human use. It safely and reproducibly releases hydromorphone for weeks in vitro and in vivo without an initial burst of drug release. Varying the thickness, diameter, and number of implants provides flexibility in the release rate and duration of release. This implantable opioid delivery device could provide a sustained subcutaneous infusion of hydromorphone to patient with cancer pain in developed and developing nations without pumps, catheters, or extensive outpatient support services. In addition, it should improve compliance and reduce concern regarding illicit diversion of opioids.

Biodegradable polymers for controlled delivery of chemotherapy with and without radiation therapy in the monkey brain

Sustained drug delivery by biodegradable polymer devices can increase the therapeutic efficacy of drugs by producing high local tissue concentrations over extended periods of time. It has been shown previously that implantation of controlled-release polymers impregnated with the nitrosourea carmustine (BCNU) extended the period of survival in rats bearing the 9L glioma compared with similar rats treated with systemically administered BCNU. This study evaluated the effect on the monkey brain of interstitial delivery of BCNU by the biodegradable polyanhydride copolymer poly[bis(p-carboxyphenoxy)propane]anhydride (PCPP) and sebacic acid (SA) in a 20:80 formulation (PCPP:SA). The effect of combining interstitial BCNU with radiation therapy was also evaluated. Eighteen male cynomolgus monkeys were randomly assigned to one of four groups: a control group; a group with implantation of empty polymer; a group with implantation of BCNU-loaded polymer; and a group with implantation of empty polymer in the right hemisphere and BCNU-loaded polymer in the left hemisphere, followed by irradiation. The effects were evaluated radiologically and histologically at specified times. A local reaction by the brain to the polymer was found, which was greater when the polymer contained BCNU. Local cerebral edema was observed radiographically on postoperative Day 14 and had resolved by Day 72. Histologically, a subacute cellular inflammatory response was seen on postoperative Day 16, which had changed to a chronic inflammatory response by Day 72. In the group with radiation therapy administered to the hemisphere bearing BCNU-loaded polymer, only localized pathological changes were detected. In all animals, brain distant from the polymer implantation site was normal. No neurological or general deleterious effects were seen in any of the animals. It is concluded that the interstitial delivery of BCNU by the polyanhydride polymer PCPP:SA is safe in the primate brain and that concomitant radiation therapy did not lead to any adverse effects. These experimental findings are important to an understanding of the clinical effects of PCPP:SA implants in treating brain diseases.

Sustained release of papaverine for the treatment of cerebral vasospasm: in vitro evaluation of release kinetics and biological activity

Cerebral vasospasm remains an unpredictable and inadequately treated complication of aneurysmal subarachnoid hemorrhage. To date, pharmacological treatment has been plagued in part by an inability to attain sufficiently high concentrations of vasodilator drug in the cerebrospinal fluid without precipitating systemic side effects such as hypotension. To circumvent this limitation of current pharmacological therapy, the authors have developed a sustained-release preparation of papaverine that can be implanted intracranially at the time of surgery for aneurysm clipping. In vitro evaluation of drug-release kinetics has demonstrated that reliable, sustained release of effective amounts of papaverine is possible. An in vitro bioassay using isolated preparations of canine basilar artery has confirmed the biological activity of this preparation. These in vitro studies are described.

The intracerebral distribution of BCNU delivered by surgically implanted biodegradable polymers

The local concentration and distribution of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) within normal brain tissue were studied following surgical implantation of biodegradable polymer containing BCNU in New Zealand White rabbits. Cylindrical discs of poly(bis(p-carboxyphenoxy)-propane:sebacic acid) copolymer in a 20:80 formulation were made containing [3H]-inulin or [3H]-BCNU labeled in the methylene hydrogens of the chloroethyl groups. These were implanted in the brains of 56 New Zealand White rabbits. The animals were sacrificed 3, 7, 14, or 21 days later and the brains were rapidly removed, frozen, and prepared for quantitative autoradiography. Autoradiographs from coronal sections bisecting the polymer were analyzed to determine both the proportion of the brain section exposed to the tracer and the local drug concentrations as a function of distance from the polymer. Tritiated BCNU was also injected directly into the brains of eight additional rabbits, and local brain concentrations were studied over time. The results of this study demonstrate that approximately 50% of the area of the brain sections was exposed to radiolabeled compound 3 days after BCNU-polymer implantation, 15% at 7 days, and less than 10% at 14 and 21 days. Polymer discs containing 600 micrograms BCNU generated 6 mM concentrations of BCNU in brain tissue 10 mm from the polymer at 3 and 7 days. Pharmacological studies demonstrated that approximately 25% of the tritium label was associated with intact BCNU 3 days following polymer implantation. Radiolabeled inulin delivered by polymer remained dispersed throughout the ipsilateral hemisphere for 14 days. Direct injection of [3H]-BCNU into brain parenchyma resulted in widely distributed tracer at 1 and 3 hours with rapid disappearance thereafter. It is concluded that local delivery of BCNU to brain tissue with this polymeric drug delivery system results in sustained high local concentrations of BCNU which may be of value in the treatment of patients with brain tumors.

Sonophoresis. I. The use of high-frequency ultrasound to enhance transdermal drug delivery

Previous attempts to use ultrasound (less than or equal to 1-MHz frequency and 1 to 3-W/cm2 intensity) to enhance transdermal drug delivery (so-called sonophoresis) have produced inconsistent results. Theoretical analysis of ultrasound propagation in tissue predicts that higher-frequency ultrasound (greater than 1 MHz) will increase the concentration of energy deposition in the stratum corneum (SC) (typically, the rate-limiting barrier to percutaneous penetration). This hypothesis was tested by comparing the passive transdermal delivery of salicylic acid with that under the influence of ultrasound at 2-, 10-, and 16-MHz frequency; measurements were performed in vivo in hairless guinea pigs. Total drug absorbed was quantified by determining the amount of salicylic acid (1) present in SC tape strips and (2) eliminated in urine. Sonophoresis for 20 min at 2 MHz caused no significant increase in salicylic acid delivery over passive diffusion; treatment with ultrasound at 10 and 16 MHz, on the other hand, significantly elevated salicylic acid transport, by 4-fold and 2.5-fold, respectively. Kinetic analysis of the sonophoretic data at 10 and 16 MHz also revealed that the diffusion lag time associated with transdermal drug delivery (TDD) was reduced. A shorter period (5 min) of sonophoresis again resulted in enhanced TDD (relative to the corresponding control) at the higher frequencies; the delivered dose, and the level of enhancement, however, were lower than those after the 20-min treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Low temperature incorporation of bovine serum albumin into a bead formed macroporous hydrophilic polymer matrix with potential for sustained release

A process of freeze-thaw polymerization involving the low temperature photopolymerization of a mixed solution of monomers and bovine serum albumin around frozen ice crystals has been used to generate a bead formed macroporous hydrophilic matrix with potential for sustained release. Beads over the size range 100-3000 microns were fabricated with surface and internal pores of between 0.7-2.6 microns whose diameter could be controlled by manipulation of the monomers to solvent ratio. Increasing both the proportion of monomers in the monomer solution and the percentage of BSA incorporated reduced the EWC of beads. The BSA release profile was characterized by an initial burst followed by a lower but sustained release lasting up to 1 month. The total cumulative release of BSA and the proportion of the incorporated BSA load subsequently released were both reduced in physiological saline compared with distilled water but enhanced by freeze drying, mild agitation and incubation at 37 degrees C.

Development of small-diameter vascular prostheses which release bioactive agents

A porous, distensible, tubular membrane which incorporates albumin and basic Fibroblast Growth Factor (bFGF), and is potentially utilizable as a bioactive small-diameter vascular prosthesis, was fabricated by a combined spraying, phase-inversion technique using a suspension of albumin and bFGF into a polyetherurethane-urea (Biomer) solution in dimethylacetamide (DMA). Scanning electron microscopy showed a material with an open-cell trabecular structure and small particles of albumin and/or bFGF entrapped in the bulk of the polyurethane trabeculae. The material released albumin and bFGF at an approximately constant rate for at least 2 weeks. The bFGF initially incorporated in the polymer remained biologically active as shown by in-vitro proliferation of human endothelial cells.

Release of human serum albumin from poly(lactide-co-glycolide) microspheres

Human serum albumin (HSA) was encapsulated in a 50:50 copolymer of DL-lactide/glycolide in the form of microspheres. These microspheres were used as a model formulation to study the feasibility of controlling the release of large proteins over a 20- to 30-day period. We show that HSA can be successfully incorporated into microspheres and released intact from these microspheres into various buffer systems at 37 degrees C. A continuous release of the protein could be achieved in physiological buffers at 37 degrees C over a 20- to 30-day period from microspheres with high protein loadings (11.6%). These results demonstrate the potential of poly(DL-lactide-co-glycolide) microspheres for continuous delivery of large proteins.

Sustained release of insulin by double-layered implant using poly(D,L-lactic acid)

This report describes the advantage of double-layered implants using low molecular weight poly(DL-lactic acid) in the sustained release of insulin. The double-layered implant consisted of a polymer matrix containing insulin and a polylactic acid layer which was coated partially on one of the surfaces of the insulin:polymer matrix. The double-layered implants were compared with single-matrix implants from the standpoint of the in vitro dissolution test and in vivo performance. In vitro release rates were controlled by changing the amount of poly(DL-lactic acid) used in the polymer layer. In an in vivo test using diabetic animals, the double-layered implants provided a sustained release of insulin for 19 d, as judged by the changes in blood glucose levels and serum insulin levels after the subcutaneous implantation.

Effect of controlled adventitial heparin delivery on smooth muscle cell proliferation following endothelial injury

Continuous intravenous infusion of heparin suppresses smooth muscle cell proliferation in rats after endothelial injury but may lead to hemorrhage and other complications. The anticoagulant property has been removed from chemically modified heparin without loss of antiproliferative effect but use of such compounds is still limited. In this study ethylene-vinyl acetate copolymer matrices containing standard and modified heparin were placed adjacent to rat carotid arteries at the time of balloon dendothelialization. After 14 days arterial occlusion by smooth muscle cell proliferation was defined. Matrix delivery of both heparin compounds effectively diminished this proliferation in comparison to controls without producing systemic anticoagulation or side effects. In addition, this mode of therapy appeared more effective than the administration of the same agents by either intravenous pumps or heparin/polymer matrices placed in a subcutaneous site distant from the injured carotid artery. Thus, heparin's inhibition of smooth muscle cell proliferation after vascular injury might be most effective within the microenvironment of the injured vessel wall, and the accelerated atherosclerosis or restenosis that often follows angioplasty and other vascular interventions might best be treated with site-specific therapy.

Sustained behavioral recovery from unilateral nigrostriatal damage produced by the controlled release of dopamine from a silicone polymer pellet placed into the denervated striatum

This study was conducted to determine if the behavioral asymmetry associated with unilateral nigrostriatal dopamine (DA) depletion could be alleviated by placing a small DA-releasing silicone polymer matrix pellet into the denervated striatum of rats. Animals that received DA-releasing pellets showed a 50% reduction in apomorphine-induced rotational behavior, and this effect persisted for the 2-month duration of the experiment. The results suggest that the controlled release of DA from an intrastriatal polymer matrix can produce a long-lasting reduction in some of the symptoms associated with DA depletion.

Effect of ultrasound on transdermal drug delivery to rats and guinea pigs

The effect of therapeutic range ultrasound (1 MHz) on skin permeation of D-mannitol, a highly polar sugar alcohol, inulin, a high molecular weight polysaccharide and physostigmine, a lipophilic anticholinesterase drug was studied in rats and guinea pigs. D-Mannitol and inulin are totally and rapidly excreted, once they have penetrated through the skin into the blood stream, permitting direct in vivo monitoring. For evaluating skin penetration of physostigmine the decrease of whole blood cholinesterase was measured. Ultrasound nearly completely eliminated the lag time usually associated with transdermal delivery of drugs. 3-5 min of ultrasound irradiation (1.5 W/cm2 continuous wave or 3 W/cm2 pulsed wave) increased the transdermal permeation of inulin and mannitol in rats by 5-20-fold within 1-2 h following ultrasound application. Ultrasound treatment also significantly increased (P less than 0.05) the inhibition of cholinesterase during the first hour after application in both physostigmine treated rats and guinea pigs: while in control guinea pigs no significant inhibition of cholinesterase could be detected during the first 2 h after application of physostigmine, the ultrasound treated group showed a 15 +/- 5% (mean +/- SEM) decrease in blood cholinesterase 1 h after ultrasound application. For physostigmine-treated rats the level of cholinesterase inhibition 1 h after ultrasound application was 53 +/- 5% in the ultrasound-treated group and 35 +/- 5% in the controls.

Enzymatically controlled drug delivery

An approach for providing feedback control for polypeptide drugs in a polymeric controlled-release system uses a trigger molecule and a polymer-bound enzyme that, in the presence of that trigger molecule, will cause an acid or a base to form. When the pH inside the polymer system changes, the solubility of the drug shifts dramatically, which changes the diffusion or dissolution driving force, and hence the release rate changes correspondingly. This concept was tested using a controlled-release system of ethylene/vinyl acetate copolymer containing insulin and immobilized glucose oxidase. The enzymatic reaction of glucose to gluconic acid reduces the pH in the polymer microenvironment. Since insulin solubility increases with decreasing pH (at physiologic pH, this is true for an insulin with an isoelectric point of 7.4 or higher), the release of insulin increases in response to glucose concentration. The feasibility of this concept has been shown using trilysyl insulin with an isoelectric point of 7.4. Multiple exposures to buffered glucose solutions over several weeks caused insulin release to reversibly increase during each exposure. Polymer-implanted diabetic rats infused with glucose solutions showed a significant increase in insulin concentration in 30 min-an effect not observed in three different sets of control rats.

Magnetically enhanced insulin release in diabetic rats

Polymer matrices containing insulin and embedded magnets were implanted subcutaneously in diabetic rats for 51 days. Passive release of insulin from the polymer resulted in a decrease in the blood glucose level. When the diabetic rats were exposed to an oscillating magnetic field, the blood glucose levels were additionally lowered by nearly 30%. No statistically significant effect in blood glucose decrease was observed in four different sets of control animals subjected to the magnetic field. Because of the very small size of the implants, they may, with additional study, provide an alternative to current modes of therapy using programmable implantable infusion pumps.

A polymeric controlled drug delivery device for peptides based on a surface desorption/diffusion mechanism

Further investigations of the mechanisms underlying continuous peptide delivery from a newly developed Accurel polypropylene/collodion device are described. This module was prepared by lumen-loading an aqueous peptide solution into a piece of microporous Accurel polypropylene tubing which was subsequently end-sealed and encapsulated by a collodion membrane. By using vasopressin (VP) as a 'model' peptide, various aspects of the known semi-reversible protein/polymer surface interaction are shown to be present for VP and polypropylene i.e. prominent adsorption on the large surface area, which follows a bimodal adsorption isotherm, but with very limited desorption, and which can be enhanced by exchange with other proteins. Based on these findings as well as the observed steady state levels of VP on 24 h immersions in a small fixed volume, the previously proposed mechanism of pseudo-zero-order release could be evaluated further: the adsorbed peptide molecules function as a stock of near-constant activity, since desorption is rather limited, and as long as rapid exchange of bound peptide by external proteins can be prevented by a sufficiently thick (greater than 60 micron) collodion membrane, a slow long-term diffusion process will take place. Under 'infinite sink' conditions, the devices gave linear cumulative release curves for weeks. The present module is a novelty among the known membrane-controlled drug-delivery reservoir systems, since it has been loaded by an unsaturated drug solution and because the encapsulating membrane is not the sole release-rate-controlling factor.

In vitro and in vivo kinetics of regulated drug release from polymer matrices by oscillating magnetic fields

The kinetics of drug release from polymer-drug matrices containing an embedded magnet was continuously monitored in vitro and in vivo. The application of an oscillating magnetic field increased the rate of drug release from the polymer matrices. Within the limits of detection the increase in release occurred immediately, remained stable for as long as the field was applied, and returned exactly to baseline upon withdrawal of the field. The increase in release was directly proportional to field amplitude. The same pattern of results were observed in vivo as in vitro, though higher strength fields were required in vivo to achieve the same effect observed in vitro.

Bioerodible polyanhydrides as drug-carrier matrices. II. Biocompatibility and chemical reactivity

The biocompatibility of bioerodible polyanhydrides and toxicology of the polymer breakdown products were assessed. Poly-[bis (p-carboxy-phenoxy) propane anhydride] (PCPP), Poly(terephthalic acid anhydride) (PTA), and their copolymers with sebacic acid were tested. The polymers did not provoke inflammatory responses in the corneas of rabbits over a six week implantation period. Subcutaneous implantation studies of PCPP in rats over a six month period showed no evidence of inflammatory cells and only slight tissue encapsulation by layers of fibroblastic cells. The degradation products of the polymers were nonmutagenic, noncytotoxic, and had a low teratogenic potential. The in vitro growth of mammalian cells on the polymers was unaffected as measured by cell morphology and cell growth rate. The chemical reactivity of the polyanhydrides with reactive model drugs, para substituted anilines, was also examined. Amides were formed when the drugs were injection molded with the polyanhydrides at 120 degrees C. However, no reaction was observed using compression molding at room temperature. No reaction occurred between the polymer and the drug during the hydrolytic degradation of the matrix at 37 degrees C.

Polymers for the controlled release of macromolecules: effect of molecular weight of ethylene-vinyl acetate copolymer

Matrices composed of ethylene-vinyl acetate copolymer (EVAc) have been used for controlled delivery of macromolecular bioactive agents. Three EVAc samples of different molecular weight (MW) were selected from solution fractionated samples. The polymer MW is a sensitive factor in affecting the release rate of bovine serum albumin (BSA); the higher the MW of EVAc, the slower the release rate. Depending on the degree of hydrophilicity of the device, the relatively hydrophilic drug particles would cause various degrees of swelling pressure upon water uptake. The relatively hydrophobic EVAc carrier would impose different degrees of restrictive force as determined by polymer MW. The interaction between the restrictive force of the carrier and the swelling pressure of the drug particles is a key factor in affecting the drug release kinetics. As a result, the selection of the polymer carrier can be used to affect the kinetics of a controlled release device.

Inhibition by diphosphonate compounds of calcification of porcine bioprosthetic heart valve cusps implanted subcutaneously in rats

Calcification limits the long-term success of heart valve bioprostheses fabricated from glutaraldehyde cross-linked porcine aortic valves. The pathophysiology of calcification of bioprostheses has been studied experimentally with subcutaneous implants of the valve cusps in rats; in this preparation, the accumulation of calcific deposits is biochemically and morphologically identical to that occurring in clinical specimens. The objective of the present study was to determine whether mineralization of bioprosthetic valve cusps (BC) subcutaneously implanted in 3-week-old male rats could be inhibited through the use of diphosphonate compounds. Ethanehydroxydiphosphonate (EHDP), administered by daily subcutaneous injection (25 mg/kg/24 hr) for 21 days inhibited calcification (BC Ca++ = 154.9 +/- 4.1), but caused somatic growth retardation and disruption of epiphyseal development. However, local administration of EHDP by osmotic pump (5 mg/kg/24 hr) implanted in direct contact with the cuspal tissue for 14 days prevented BC calcification (BC CA++ = 4.3 +/- 0.7) without adverse effects. Furthermore, EHDP given by osmotic pump had a prolonged effect on reducing calcification, as demonstrated by implants harvested 21 days (BC CA++ = 12.2 +/- 6.4) after the drug supply was exhausted. Finally, BC preincubated in aminopropanehydroxydiphosphonate for 24 hr before 21 day implantation underwent less calcification (CA++ = 24.2 +/- 7.4) than control valves (BC CA++ 126.6 +/- 7.5) with no adverse effects. We conclude that diphosphonates inhibit BC calcification, and that adverse effects of systemic therapy can be avoided by local administration.

The role of polymer matrix structure and interparticle interactions in diffusion-limited drug release

A lattice random-walk model is used to simulate diffusion in a porous polymer. This model may be useful for the practical design of drug-release systems. Both interacting and noninteracting particles (random walkers) were allowed to diffuse through a pore with a single exit hole. It was found that the specific interactions among the diffusing particles have little influence on the overall release rate. Diffusion through more complicated structures was investigated by simulating the diffusion of particles through two pores connected by a constricted channel whose length and width were varied. The overall rate of release was found to be proportional to the width of the constricted channel. When the length of the channel was greater than or equal to the length of the pore, the rate of release was also inversely proportional to the channel length. From a practical standpoint, release rates can be decreased (and times for release increased) by one or two orders of magnitude by decreasing the width and expanding the length of the interconnecting channels in the polymer matrix.

Regulation of drug release from polymer matrices by oscillating magnetic fields

The reproducible regulation of release of a macromolecule (bovine serum albumin) from biocompatible polymer systems has been demonstrated. Small magnetic spheres or cylindrical magnets were embedded within the polymer matrix which was then subjected to an oscillating magnetic field. In this fashion baseline release rates could be increased 5- to 10-fold with 5-10% standard error. Parameters critical to the regulation of this release included the position, orientation, and magnetic strength of the embedded objects and the amplitude and frequency of the applied magnetic field. Scanning electronmicrographs of the polymer matrix surface reveal that a gap, approximately 100/microns wide, is formed between the embedded object and adjacent polymer material after repeated exposure to an oscillating magnetic field.