The present invention generally falls within the therapeutic field and more particularly within the field of therapy for neoplastic pathologies.

The enzyme creatine kinase (EC 2.7.3.2, CK) plays an essential role in the energy metabo- lism of cells (Saks et al 1975; Seraydarian and Abbott 1976). The said enzyme is ubiquitous and capable of transferring phosphate groups from ATP to creatine and from phos- phocreatine to ADP, allowing for the recharging of ATP. In the proximity of mitochondria, the enzyme creatine kinase catalyzes the phosphorylation of creatine, thereby generating ADP and phosphocreatine. In the cell areas that require more energy, creatine kinase transfers the phosphate group from phosphocreatine to ADP to restore ATP, thus supporting the cell's energy requirements. The enzyme creatine kinase has a molecular weight of approximately 60KDa and is made of four subunits: two cytosolic, M-CK and B-CK, and two mitochondrial, "ubiquitous" uMt-CK and "sarcomeric" sMt-CK (Joseph et al 1997; Griffiths 1982).

The two cytosolic subunits are the M (from muscle) subunit and the B (from brain) subu- nit. The genes that express them are distributed on two different chromosomes: chromosome 14q32 for subunit B and chromosome 19ql3 for subunit M (Stallings et al 1988). The combination thereof leads to three different isoforms expressed in the districts that require more energy (Walliman et al 1992; Wyss et al 1992): the muscle isoenzyme (CK- MM), the heart isoenzyme (CK-MB) and the brain isoenzyme (CK-BB). The three isoenzymes catalyze the same reaction, namely the reversible transfer of the ATP gamma- phosphate group to the guanidine group of creatine, thereby obtaining phosphocreatine and ADP.

A number of studies have demonstrated that the activity of the creatine kinase enzyme is involved in the growth of cancer (Joseph et al 1997; Li et al 2013; Zhang et al 2009), with its several isoenzymes having different roles in the neoplastic tissues (Kristensen et al 1999). For instance, the brain isoenzyme (CK-BB) was shown to be involved in the tumorigenesis process (Kaddurah-Daouk et al 1990).

A few chemical inhibitors of the creatine kinase enzyme are known. lodoacetamide is an alkylating agent that provides complete and irreversible inhibition on the enzyme (Rodri- guez et al 2003; Ren et al 2009). The 3-butyl-l-phenyl-2-(phenyltelluro)oct-en-l-one is an α-β-unsaturated ketone used in various industrial productions and as an anti-detonating additive in petrol (Fairhill 1969; Yarema and Curry 2005). It is a competitive inhibitor of the creatine kinase enzyme (De Andrade et al 2010; De Andrade et al 2012). In consideration of the role played by creatine kinase in the development of cancer, the use of cyclocreatine as treatment against cancer has been studied (Martin et al 1994). This molecule has been found suitable for inhibiting tumor cell lines that express a high level of creatine kinase (Lillie et al 1993). The antitumor action of other creatine analogues has also been assessed (Bergnes et al 1996).

Creatine derivatives studied so far for the inhibition of creatine kinase, however, present the inconvenience of a non-modulable and irreversible inhibition of the enzyme.

In order to overcome this and other problems of the prior art, the present invention now provides a new antitumor agent which can inhibit the creatine kinase enzyme in a reversible and modulable manner, (Boc) ₂ -creatine. These properties make (Boc) ₂ -creatine particularly advantageous in cancer therapy since it can inhibit cell growth in a modulable and reversible manner, blocking the creatine kinase enzyme, the cell's source of energy. An important aspect is that the block caused by (Boc) ₂ -creatine can be modulated according to need, as far as the total block of the enzymatic activity.

(Boc) ₂ -creatine is a creatine derivative obtained by chemical synthesis (Millo et al 2012; Garbati et al 2013), which presents 2 t-Boc groups (tert-butoxycarbonyl) on the guanidine group. The formula of the structure of (Boc) ₂ -creatine is shown below:

The (Boc) ₂ -creatine used by the present inventors in the experiments described below was synthesized according to the method described in the Italian patent application TO2012 A001098 filed on 18 December 2012.

The present inventors have checked that (Boc) ₂ -creatine is effective as a modulable blocking agent of the creatine kinase enzyme. This action on the creatine kinase enzyme makes (Boc) ₂ -creatine a particularly promising molecule as a drug with an antineoplastic action, since the enzyme can be subjected to the desired degree of inhibition by modifying the concentration of the (Boc) ₂ -creatine.

To the best of the inventors' knowledge, no therapeutic use of (Boc) ₂ -creatine has yet been described. A first aspect of the present invention is therefore (Boc) ₂ -creatine for use as a medicament.

A second aspect of the present invention is (Boc) ₂ -creatine for use in the treatment of tumor diseases. For these purposes, (Boc) ₂ -creatine can be administered alone or in combination with one or more other antitumor agents. Non-limiting examples of other antitumor agents that can be used in combination with (Boc) ₂ -creatine include: alkylating agents such as cisplatin and melphalan, derivatives of cyclophosphamide and carmustine. (Boc) ₂ -creatine or the combination of (Boc) ₂ -creatine with one or more other antitumor agents can be adminis- tered to the patient in a suitable form of pharmaceutical dosage comprising conventional excipients and vehicles of pharmaceutical technology and by various means of administration, including, without limitation, the topical, enteral and parenteral routes. The amount of (Boc) ₂ -creatine administered, optionally in combination with one or more other antitumor agents, is a therapeutically effective amount, i.e. capable of producing in the patient the desired effect of inhibiting the creatine kinase enzyme and therefore of inhibiting or reducing the cell proliferation. According to the experiments carried out by the present inventors and illustrated below, a therapeutically effective amount is a quantity which can give a concentration of (Boc) ₂ -creatine of between 0.25 mM and < 2 mM, preferably included in the range between 0.25 mM and 1.9 mM, or between 0.25 mM and 1.7 mM, or 0.25 mM and 1.5 mM, or 25 mM and 1.3 mM, more preferably between 0.25 mM and 1 mM. Of course, the effective amount of (Boc)2-creatine to be administered will depend on various factors such as, for example, the patient's weight and age, the type and severity of the dis- ease to be treated, other medical parameters, as well as the desired purpose of the treatment.

The examples below are given for merely illustrative purposes and must not be considered as limiting the scope of the invention.

Examples METHODS Activity of the creatine kinase enzyme in the presence of (Boc)?-creatine

The activity of the creatine kinase enzyme was measured by the enzymatic coupling method (Bergmeyer 1974). In this method, the ATP hydrolysis is coupled with the NADH oxidation, which is monitored, by means of a spectrophotometer, by the decrease in absorb- ance at 340 nm (ε340 = 6.22 mM ^"1 cm ^"1 ).

Dosage of the creatine kinase enzyme and determination of its kinetic constants in the presence o f (Boc)r-creatine The dosage of the creatine kinase was carried out with the spectrophotometer at 340 nm, following the oxidation of the pyridine coenzyme NADH, which is expressed as a decrease in the absorbance. To be able to carry out the dosage, the reaction of the creatine kinase was combined with two other enzymatic reactions catalyzed respectively by the pyruvate kinase and by the lactate dehydrogenase (Bergmeyer 1974). This method, defined as the coupled method, is necessary because both the substrata and the products of the creatine kinase are not directly visible to the spectrophotometer. In particular, the dosage solution contained the following compounds: 100 mM Tris HC1, pH 7.5, 2 mM MgCl ₂ , 75 rnivr C1, 2 mM ATP, 0.5 mM phosphoenolpyruvate, 0.16 mM NADH, 10 IU of pyruvate kinase and of lactate dehydrogenase, in a final volume of 1 ml. The reaction begins by the addition of 2 mM of creatine. The dosage is carried out continuously, i.e. the enzyme activity is followed in real time during the development of the reaction. The dosage trend is monitored by a register connected to the spectrophotometer. The reactions take place in the cuvette, as illustrated in figure 1.

To verify the inhibiting effect of (Boc) ₂ -creatine, concentrations between 0.25 and 2 mM were added to the dosage solution, before the addition of the creatine.

To identify the type of inhibition, the maximum kinetic speed (V _max ) and the Michaelis Menten constant (K _m ) were measured. To be able to carry out the experiment, for each concentration of (Boc) ₂ -creatine, the dosage was carried out in the presence of different substratum concentrations (creatine). The data were analyzed with the Lineweaver-Burk double-reciprocal plot.

RESULTS

From figure 2 it can be observed that the maximum inhibition (95%) of the creatine kinase is obtained with 1 mM of (Boc) ₂ -creatine. However, already at a concentration of (Boc) ₂ - creatine equal to 0.25 mM an inhibition of 21% can be observed, which progressively increases to 39% and to 75% when the concentration of (Boc) ₂ -creatine used is respectively 0.5 mM or 0.75 mM. These data confirm that the inhibition of the enzyme can be modulated, using different concentrations of (Boc) ₂ -creatine.

To understand what type of inhibition the (Boc) ₂ -creatine causes to the creatine kinase enzyme, other experiments were carried out varying both the concentration of the (Boc) ₂ - creatine (0 mM; 5 mM; ImM) and the concentration of creatine (the substratum of the enzyme; between 0 and 2 mM). As can be observed in figure 3, the inhibition caused by (Boc) ₂ -creatine is of the competitive type because it does not interfere with the maximum speed (V _max ; intercept of the y axis) of the enzymatic catalysis, but it affects the capacity to bind the substratum to the active site of the enzyme (K _m ; intercept of the x axis). In other words, the (Boc) ₂ -creatine competes with the creatine to bind to the site of the creatine kinase enzyme and thus slows down its activity.

The graph in figure 3 allows for identifying two of the most common kinetic enzyme pa- rameters, Maximum speed (V _max ) and the Michaelis-Menten Constant (K _m ). The first indicates the maximum speed at which the enzyme metabolizes its own substratum over a determined period of time. The second indicates the affinity of the enzyme for the substratum, i.e. the capacity to bind the creatine to the active site. The V _max is represented by the reciprocal of the intercept of the Y axis, while the K _m is represented by the negative recip- rocal of the intercept of the X axis. As can be observed, even if the concentration of the (Boc) ₂ -creatine varies, the value of the intercept of the Y axis does not change, while the value of the intercept of the X axis does change, increasing in proportion to the concentration of (Boc) ₂ -creatine. This suggests that (Boc) ₂ -creatine competes with the creatine to bind to the active site of the creatine kinase enzyme, causing a competitive type inhibition.

Discussion of the results

The above-described results show that (Boc) ₂ -creatine is a competitive inhibitor of the creatine kinase enzyme. It binds to the active site of the enzyme, thus preventing the binding of the natural substratum, the creatine. This action results in blocking the enzymatic activity with a consequent cytostatic effect and block of the progression of all the phases of the cell cycle.

Of course, high concentrations of (Boc) ₂ -creatine will have a cytotoxic effect. In fact, slic- es of mouse hippocampus incubated with 2mM (Boc) ₂ -creatine for 3 hours at 36°C show no vital signs. Since tumor development is due to an uncontrolled cellular progression, a molecule which can control the main source of cell energy gives a valid means of control- ling cell proliferation.

From the experiments on enzymatic kinetics carried out by the present inventors, it has been deduced that the inhibition of the enzyme takes place in proportion to the concentra- tion of (Boc) ₂ -creatine present. In fact, at a concentration of (Boc) ₂ -creatine equal to 0.25 mM there is an inhibition of 21% and this inhibition increases in proportion to the increase in the concentration of (Boc) ₂ -creatine up to the maximum inhibition (95%) of the creatine kinase which is obtained when the concentration of the molecule rises to 1 mM. REFERENCES

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