**8. Drug eluting stents**

464 Biomaterials – Physics and Chemistry

and no animal had a distal obstructed ureter Due to retained stent pieces. Properties such as stent softness from these biodegradable stents may improve patient comfort. Clinical studies

One of the most common stent coatings is hydrogel, which consists of hydrophilic polymers that absorb water and increase lubricity and elasticity.(Marmieri, Pettenati et al. 1996; John, Rajpurkar et al. 2007) These properties facilitate stent placement, making the device rigid and easily maneuverable in its dry state, but once exposed to urine, the hydrogel begins to absorb and trap water in its polyanionic structure, causing it to soften and theoretically increase patient comfort. Data on encrustation and infection are less convincing, as hydrogel has been shown to both reduce (Gorman, Tunney et al. 1998) and increase encrustation and biofilm formation (Desgrandchamps, Moulinier et al. 1997). Hydrogels have been used in an attempt to soak and retain antibiotics but an *in vitro* study did not show increased efficacy of bacterial killing compared to non-antibiotic soaked hydrogel coated stents (John, Rajpurkar

Glycosaminoglycan (GAG), a normal constituent of urine, is a natural inhibitor of crystallization. Other novel stent coatings include pentosan polysulfate (Zupkas, Parsons et al. 2000) (a member of the Glycosaminoglycan family a normal constituent of urine and a natural inhibitor of crystallization), phosphorylcholine (Stickler, Evans et al. 2002) (a constituent of human erythrocytes that mimics a natural lipid membrane), and polyvinyl pyrrolidone (Tunney and Gorman 2002) (a hydrophilic coating, similar to hydrogel, that

Attempts to reduce encrustation have included other stent coatings, such as the bacterial enzyme, oxalate decarboxylase, which has been shown to decrease encrustation in silicone discs placed in rabbit bladders.(Watterson, Cadieux et al. 2003) A novel coating of mPEG-DOPA3, a natural constituent produced by mussels that produces strong adhesive properties, also has the ability to avoid biofouling in the environment. The polyethyelene (PEG) component provides the antifouling property while the DOPA3 provides the adherence that PEG lacks on its own. Adherence of these combined compounds on silicone disks has resulted in a strong ability to resist bacterial adherence and growth in vitro.(Ko 2007) Further development of this type of coating was studied *in vivo* using a rabbit *E. coli* cystitis model (Pechey, Elwood et al. 2009). This study showed that the anti-adhesive coating was successful at inhibiting bacterial adhesion and decreased the incidence of

infection, however it was unable to prevent non-bacterial mediated encrustation.

Plasma deposited diamond like carbon coatings have been used to coat stents in an attempt to prevent encrustation (Laube, Kleinen et al. 2007). *In vitro* experiments have shown a 30% decrease in encrustation of these stents in artificial urine compared to the non-coated controls. Ongoing clinical trials appear to indicate a further enhancement of these results *in vivo*, however a mechanism for this needs to be elucidated. Encrustation of ureteral stents remains one of the most common problems associated with ureteral stenting and more research will need to be done for an optimal stent design which resists the deposition of

In vascular medicine, the anticoagulant heparin has been shown to inhibit bacterial attachment to venuous catheters (Ruggieri, Hanno et al. 1987; Appelgren, Ransjo et al. 1996), which has been attributed to its highly negative charge. Similarly, effects of heparin have

will be necessary to determine if biodegradable stents are more comfortable.

**7. Stent coatings** 

et al. 2007).

absorbs water).

bacteria, minerals and proteins.

The most serious complications of long term stenting involve infection triggered by bacterial adherance and biofilm formation on the surfaces of stents as well as patient discomfort due to stent placement. Much research has gone into the prevention of infection, and the most promising results have come from drug eluting stents. Triclosan is an antimicrobial used in many products including soap, surgical scrub, toothpaste, and mouthwash. It inhibits the highly conserved bacterial enoyl-ACP reductase, which is responsible for fatty acid synthesis and cell growth. Cadieux *et al*. reported that, compared to control stents, triclosanloaded stents implanted in rabbit bladders infected with *Proteus mirabilis* were associated with significantly fewer urinary tract infections.(Cadieux, Chew et al. 2006) Chew *et al.* have shown that bacterial adherence to triclosan eluting stents is markedly reduced compared to regular stents.(Chew, Cadieux et al. 2006) These studies indicate that human clinical trials involving these stents are warranted.

Ureteral stents may also be loaded with pharmaceuticals to aid patient comfort, and to prevent encrustation. Irritative and painful stent symptoms have traditionally been managed with oral medications such as anticholinergics and analgesics, or even by stent removal. Drug-eluting stents release a medication that acts locally on the bladder to decrease irritation and pain. In an attempt to determine which medication might improve stent-related symptoms, Beiko *et al*. intravesically instilled 3 different medications into the bladder of 40 patients who received a ureteral stent at the time of shockwave lithotripsy.(Beiko, Watterson et al. 2004) Intravesical ketorolac significantly reduced flank pain scores following stent insertion compared to lidocaine or oxybutynin following SWL. A ketorolac-eluting ureteral stent was designed and shown to produce the highest levels of ketorolac in the ureteral tissues in an porcine model (Chew, Davoudi et al. 2010). The levels of ketorolac in the ureter were 11 fold of that found in the serum thereby reducing potential systemic side effects while delivering medication directly to the target area. The stent was biocompatible and systemic levels of ketorolac were negligible. A double-blinded prospective randomized controlled trial comparing ketorolac-eluting ureteral stents to

Biomaterials in Urology - Beyond Drug Eluting

inhibition of further bacterial proliferation.

**10. Current stent biomaterial design** 

upregulate resistance genes once inside the biofilm (Lewis 2005).

and Degradable - A Rational Approach to Ureteral Stent Design 467

These components are believed to facilitate bacterial adhesion leading to bacterial colonization, proliferation, and biofilm formation with subsequent infection. Once a biofilm has formed, this environment facilitates recurrent infection and eradication of bacteria is difficult. Bacteria embedded within the biofilm change to a low metabolic state and undergo a low replication rate, thus rendering antibiotics (which are most effective against bacteria in high metabolic states and undergoing replication) ineffective. In many cases, embedded bacteria are also protected since antibiotics cannot penetrate the biofilm and the protecting exopolysaccharide layer excreted by the bacteria onto its surface. Thirdly, bacteria can

Aside from biofilm formation and infection, another symptom associated with patient morbidity caused by indwelling ureteral stents is device encrustation. Stent encrustation can be idiopathic and caused by calcium oxalate crystals. In other instances, stent encrustation can be attributed to the presence of urease producing bacteria, which break down urinary ammonia into ammonium (thus effectively taking a hydrogen ion), which results in a rise in urinary pH and crystallization of magnesium, ammonium and phosphate ions. These crystals then adhere to the surface of the stent via the interaction with components of the conditioning film. The conditioning film on the stent surface is considered to be a great contributor to bacterial associated encrustation because it facilitates bacterial adhesion and crystal adhesion to the stent surface. In addition to this, the conditioning film has also been implicated in idiopathic encrustation (in the absence of bacteria) of the stent with calcium oxalate crystals. As such, certain conditioning film components have been proposed to be able to bind minerals from the urine, forming a nidus for crystal growth and device encrustation. To date, we have identified 3 potential targets to interrupt the sequence of events involved in the evolution of stent encrustation and infection: 1) preventing conditioning film formation, 2) preventing initial adherence and encrustation and 3)

Over the years, attempts have been made at preventing stent associated symptoms by targeting either bacterial adhesion and encrustation or inhibition of bacterial proliferation. Drug eluting technology to prevent bacterial adhesion has previously been used in a triclosan-eluting ureteral stent. Triclosan is an antimicrobial found in over 800 commercially available products such as soaps, hand scrubs, and toothpaste. This stent proved to be successful at eliminating bacterial loads *in vitro* (Chew, Cadieux et al. 2006) as well as a *Proteus mirabilis* urinary tract infection in a rabbit model (Cadieux, Chew et al. 2006), but did not show any significant differences in long term clinical trials (Cadieux, Chew et al. 2009). Similarly a heparin- coated stent was designed to prevent bacterial adhesion given the material's highly negative charge. This stent was shown to decrease encrustation in patients (Hildebrandt, Sayyad et al. 2001; Riedl, Witkowski et al. 2002; Cauda, Cauda et al. 2008), however was unable to prevent bacterial adhesion (Lange, Elwood et al. 2009). The use of diamond-like amorphous carbon as a coating on stents is a new technology that has shown some promise in terms of inhibiting encrustation (Laube, Bradenahl et al. 2006; Laube, Kleinen et al. 2007), however experiments aimed at determining its ability to inhibit bacterial adhesion is lacking. One of the drawbacks of these new technologies is the fact that they are susceptible to blockage by the deposition of the urinary conditioning film, which covers any coating and blocks elution of drugs from the stent, rendering it ineffective and promoting

bacterial adhesion and encrustation via mechanisms discussed above.

controls showed no difference in pain scores except in young males who had less symptoms with the ketorolac eluting stent (Krambeck, Walsh et al. 2010).

Liatsikos *et al* have tested paclitaxel eluting metal stents in the pig ureter to examine the tissue effects and stricture formation.(Liatsikos, Karnabatidis et al. 2007) Paclitaxel eluting stents produced less ureteral inflammation and hyperplasia of the surrounding tissue compared to the bare metal stents. Ureteral patency was lost in the control stents and maintained by the Paclitaxel eluting stents. These studies were carried out over a 21 day period and require further validation via long term animal trials.

Stent encrustation worsens with increased indwelling time and concurrent infection with urease-producing organisms. Oxalate is normally broken down in the gastrointestinal tract by the enzyme oxalate decarboxylase, which is found in a commensal organism *Oxalobacter formigenes*. Oxalate that escapes degradation and fecal excretion is absorbed into the bloodstream and filtered in the kidneys where, under certain conditions, it can combine with calcium to form calcium oxalate stones. Watterson *et al*. coated silicone disks with oxalate decarboxylase and implanted these into rabbit bladders.(Watterson, Cadieux et al. 2003) After 30 days, the oxalate decarboxylase-coated disks were significantly less encrusted than control disks. Coating ureteral stents with such an enzyme could theoretically prevent encrustation as the stent would elute an enzyme to degrade urinary oxalate.

#### **9. Identifying potential targets in stent design**

When considering the design of new indwelling ureteral devices such as stents or catheters, the sequential steps triggering a given side effect should be taken into consideration, however this has been complicated by the complexity of mechanisms involved. Rational drug design hypothesizes that the alteration of a biological target has therapeutic value and forms the basis for the invention of new medications predicated on the identification and knowledge of a specific biological target. The first step involves turning to basic science and considering the molecular and biochemical pathways involved in the condition to identify specific targets for drug design. Once a target has been identified, its molecular structure is determined and a suitable drug that will alter it in a favorable manner is designed. Usually the target is a key molecule in a metabolic or signaling pathway specific to a disease condition or pathology (Mandal, Moudgil et al. 2009).

We believe that the same principals can also be applied to the design of ureteral stents, as the current stent designs have failed to live up to their expectations in the complex environment of the urinary tract. Given the fact that the mechanisms causing stent symptoms are unknown makes it difficult to identify a key target in the context of rational drug design to relieve patient symptoms. The identification of such a target in the urinary tract would be beneficial, as it will allow for the reduction or elimination of stent symptoms by targeting a single mechanism. However, in order for that to become a possibility, key steps in the mechanisms surrounding stent-related symptoms need to be identified to allow for their inhibition.

Although identifying a single receptor or enzyme target in the development of stent encrustation and infection is unlikely, a more solid understanding of the mechanism involved in this process is required. Several processes occur following stent insertion and the cumulative effect can result in stent associated symptoms suffered by the patient. It has been well documented that a urinary conditioning film deposits on the stent surface shortly following device insertion that consists of urinary components (Tieszer, Reid et al. 1998).

controls showed no difference in pain scores except in young males who had less symptoms

Liatsikos *et al* have tested paclitaxel eluting metal stents in the pig ureter to examine the tissue effects and stricture formation.(Liatsikos, Karnabatidis et al. 2007) Paclitaxel eluting stents produced less ureteral inflammation and hyperplasia of the surrounding tissue compared to the bare metal stents. Ureteral patency was lost in the control stents and maintained by the Paclitaxel eluting stents. These studies were carried out over a 21 day

Stent encrustation worsens with increased indwelling time and concurrent infection with urease-producing organisms. Oxalate is normally broken down in the gastrointestinal tract by the enzyme oxalate decarboxylase, which is found in a commensal organism *Oxalobacter formigenes*. Oxalate that escapes degradation and fecal excretion is absorbed into the bloodstream and filtered in the kidneys where, under certain conditions, it can combine with calcium to form calcium oxalate stones. Watterson *et al*. coated silicone disks with oxalate decarboxylase and implanted these into rabbit bladders.(Watterson, Cadieux et al. 2003) After 30 days, the oxalate decarboxylase-coated disks were significantly less encrusted than control disks. Coating ureteral stents with such an enzyme could theoretically prevent

When considering the design of new indwelling ureteral devices such as stents or catheters, the sequential steps triggering a given side effect should be taken into consideration, however this has been complicated by the complexity of mechanisms involved. Rational drug design hypothesizes that the alteration of a biological target has therapeutic value and forms the basis for the invention of new medications predicated on the identification and knowledge of a specific biological target. The first step involves turning to basic science and considering the molecular and biochemical pathways involved in the condition to identify specific targets for drug design. Once a target has been identified, its molecular structure is determined and a suitable drug that will alter it in a favorable manner is designed. Usually the target is a key molecule in a metabolic or signaling pathway specific to a disease

We believe that the same principals can also be applied to the design of ureteral stents, as the current stent designs have failed to live up to their expectations in the complex environment of the urinary tract. Given the fact that the mechanisms causing stent symptoms are unknown makes it difficult to identify a key target in the context of rational drug design to relieve patient symptoms. The identification of such a target in the urinary tract would be beneficial, as it will allow for the reduction or elimination of stent symptoms by targeting a single mechanism. However, in order for that to become a possibility, key steps in the mechanisms surrounding stent-related symptoms need to be identified to allow

Although identifying a single receptor or enzyme target in the development of stent encrustation and infection is unlikely, a more solid understanding of the mechanism involved in this process is required. Several processes occur following stent insertion and the cumulative effect can result in stent associated symptoms suffered by the patient. It has been well documented that a urinary conditioning film deposits on the stent surface shortly following device insertion that consists of urinary components (Tieszer, Reid et al. 1998).

with the ketorolac eluting stent (Krambeck, Walsh et al. 2010).

period and require further validation via long term animal trials.

encrustation as the stent would elute an enzyme to degrade urinary oxalate.

**9. Identifying potential targets in stent design** 

condition or pathology (Mandal, Moudgil et al. 2009).

for their inhibition.

These components are believed to facilitate bacterial adhesion leading to bacterial colonization, proliferation, and biofilm formation with subsequent infection. Once a biofilm has formed, this environment facilitates recurrent infection and eradication of bacteria is difficult. Bacteria embedded within the biofilm change to a low metabolic state and undergo a low replication rate, thus rendering antibiotics (which are most effective against bacteria in high metabolic states and undergoing replication) ineffective. In many cases, embedded bacteria are also protected since antibiotics cannot penetrate the biofilm and the protecting exopolysaccharide layer excreted by the bacteria onto its surface. Thirdly, bacteria can upregulate resistance genes once inside the biofilm (Lewis 2005).

Aside from biofilm formation and infection, another symptom associated with patient morbidity caused by indwelling ureteral stents is device encrustation. Stent encrustation can be idiopathic and caused by calcium oxalate crystals. In other instances, stent encrustation can be attributed to the presence of urease producing bacteria, which break down urinary ammonia into ammonium (thus effectively taking a hydrogen ion), which results in a rise in urinary pH and crystallization of magnesium, ammonium and phosphate ions. These crystals then adhere to the surface of the stent via the interaction with components of the conditioning film. The conditioning film on the stent surface is considered to be a great contributor to bacterial associated encrustation because it facilitates bacterial adhesion and crystal adhesion to the stent surface. In addition to this, the conditioning film has also been implicated in idiopathic encrustation (in the absence of bacteria) of the stent with calcium oxalate crystals. As such, certain conditioning film components have been proposed to be able to bind minerals from the urine, forming a nidus for crystal growth and device encrustation. To date, we have identified 3 potential targets to interrupt the sequence of events involved in the evolution of stent encrustation and infection: 1) preventing conditioning film formation, 2) preventing initial adherence and encrustation and 3) inhibition of further bacterial proliferation.
