## 1 Introduction  
Bone metastasis is prevalent in many cancers, especially breast, prostate or lung cancer, the most common neoplasms in the world today [1, 2]. Cancer patients with bone metastasis are exposed to numerous skeletal disorders, such as unexpected pathological fractures, serious hypercalcaemia or severe bone pain which is difficult to relieve [1]. Until now, the first-line treatment for bone metastasis has administered bisphosphonates (BPs) [1, 2, 3, 4]. Their mechanism of action is now more clear [5, 6, 7]. It is commonly known that they inhibit bone resorption by suppressing osteoclastogenesis and osteoclast activity via the farnesyl pyrophosphate synthase enzyme (FPPS) in the mevalonic acid pathway [6, 7]. Moreover, recent research shows that BPs may inhibit bone tumour growth and tumour cell invasion in the extracellular matrix. These studies suggest a preventive role played by BPs in tumour metastasis in bone tissues [8, 9].  
Among the most effective BPs in bone metastasis treatment are: clodronate (CLO), pamidronate, ibandronate and zoledronic acid [2, 5, 10]. They are administered to patients via two routes--oral or intravenous--though unfortunately they can cause some side effects, such as an acute systemic inflammatory reaction, ocular inflammation, nephrotic syndrome or electrolyte imbalance [11]. Moreover, when applied orally, the bioavailability of these drugs is very low and often insufficient. Thus, it seems reasonable to deliver BPs locally and as a consequence accelerate their local bioavailability.  
It should be emphasized that the therapeutic efficacy of BPs administered by standard methods is also limited. In recent years, unconventional macromolecular drug delivery systems (DDS) have became the focus of interest [12, 13, 14]. Polymeric DDS exhibit unique pharmacokinetics, distribution and pharmacological efficacy. Numerous BP delivery systems (BPDDS) have been investigated [15, 16, 17, 18, 19, 20, 21, 22, 23]. They include dendrimeric polymers, hydrogels, liposomes, nanocapsules, nanospheres and macromolecular conjugates. One particularly interesting kind of BPDDS comprises biomaterials used as orthopaedic implants [15, 16].  
Aliphatic or cycloaliphatic polyurethanes (PUs) demonstrate good biodegradability and biocompatibility in human tissues. These attributes make them advantageous and extremely useful for the technology of controlled DDS [14, 24].  
Until now, the studies into BPDDS have been carried out mostly using animal models, with only a few exceptions employing human clinical trials. As such, the preparation of novel BPDDS is especially interesting for the pharmaceutical industry and medicine in general.  
Moreover, it is important to know that BPs exhibit a strong affinity to nanocrystalline hydroxyapatite [25, 26]. It has been shown that they may strongly adsorb on the apatite surface by an ion-exchange mechanism between phosphonate groups from BPs and phosphate ions from hydroxyapatite (HA) [27]. Several authors have reported that some BPs may also adsorb on HA through surface binding of phosphonate groups and Ca\({}^{2+}\) sites of HA [25, 28]. Therefore, the use of HA nanoparticles as the delivery system of BPs has been widely studied [29, 30].  
The main aim of our study has been to prepare composite CLO DDS using biodegradable PUs hydroxyapatite (HA) as components. The structures and chemical compositions of the new biomaterials were investigated and discussed based on the results obtained.