Abstract:
A mechanism in the body is used to drive a piston rod and comprises means for guiding the movement of the piston rod relatively to the body while delivering the drug. Further means are provided for releasing the piston rod from the means for guiding the movement, thus enabling a reset of the piston rod to a start position, especially after the receptacle has been emptied and is to be refilled. The device is constructed in such a manner that a reset of the piston rod is facilitated by a stop element inhibiting a reversal of the movement of the piston rod when being reset.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a U.S. National Phase Application pursuant to 35 U.S.C. §371 of International Application No. PCT/EP2010/054342 filed Mar. 31, 2010, which claims priority to European Patent Application No. 09004665.7 filed on Mar. 31, 2009 and U.S. Provisional Patent Application No. 61/169,852 filed on Apr. 16, 2009. The entire disclosure contents of these applications are herewith incorporated by reference into the present application. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a drug delivery device, particularly to a portable pen-type drug injector. 
       BACKGROUND 
       [0003]    Portable drug delivery devices are used for the administration of a medicinal fluid or drug. A drug injection device is especially useful in the shape of a pen. A dose of a drug is delivered by means of a drive mechanism using a piston driven by a piston rod. The device may be refillable, especially by means of exchangeable cartridges, which contain the drug to be injected and are inserted in the body of the injection pen. In this case the piston can be arranged within the cartridge, while the piston rod is a component of the drive mechanism. When an emptied cartridge is substituted with a new one, the drive mechanism has to be reset. 
         [0004]    EP 1 923 083 A1 describes a drug delivery device in the shape of an injection pen having a drive mechanism, which allows to deliver a plurality of different prescribed doses. 
         [0005]    WO 2008/074897 A1 describes a syringe device in the shape of a pen. It comprises a container for a drug and a piston for the delivery of the drug, further a housing having a helical thread, a dose dial sleeve having a helical thread engaged with the helical thread of the housing, a drive sleeve detachably connected to the dose dial sleeve and a clutch means located between the dose dial sleeve and the drive sleeve. When the dose dial sleeve and the drive sleeve are coupled via the clutch means, both are allowed to rotate with respect to the housing. When the dose dial sleeve and the drive sleeve are decoupled, rotation of the dose dial sleeve with respect to the housing is allowed whilst rotation of the drive sleeve with respect to the housing is not allowed, whereby axial movement of the drive sleeve is allowed so that a force is transferred in the longitudinal direction to a piston rod for drug delivery. 
       SUMMARY 
       [0006]    It is an object of the present invention to disclose a drug delivery device comprising a mechanism with a means that facilitates a reset of the mechanism. 
         [0007]    This object is achieved with the drug delivery device according to claim  1 . Further aspects and variations of the invention derive from the depending claims. 
         [0008]    The drug delivery device comprises a housing or body with a receptacle for a drug. The body contains a piston rod, which is provided for delivering the drug out of the receptacle. A mechanism in the body is used to drive the piston rod and comprises means for guiding the movement of the piston rod relatively to the body while delivering the drug. Means are provided for releasing the piston rod from the means for guiding the movement, thus enabling a reset of the piston rod to a start position, especially after the receptacle has been emptied and is to be refilled. The device is constructed in such a manner that a reset of the piston rod is facilitated by means of a stop element. 
         [0009]    The term “drug”, as used herein, means a pharmaceutical formulation containing at least one pharmaceutically active compound, 
         [0010]    wherein in one embodiment the pharmaceutically active compound has a molecular weight up to 1500 Da and/or is a peptide, a proteine, a polysaccharide, a vaccine, a DNA, a RNA, a antibody, an enzyme, an antibody, a hormone or an oligonucleotide, or a mixture of the above-mentioned pharmaceutically active compound, 
         [0011]    wherein in a further embodiment the pharmaceutically active compound is useful for the treatment and/or prophylaxis of diabetes mellitus or complications associated with diabetes mellitus such as diabetic retinopathy, thromboembolism disorders such as deep vein or pulmonary thromboembolism, acute coronary syndrome (ACS), angina, myocardial infarction, cancer, macular degeneration, inflammation, hay fever, atherosclerosis and/or rheumatoid arthritis, 
         [0012]    wherein in a further embodiment the pharmaceutically active compound comprises at least one peptide for the treatment and/or prophylaxis of diabetes mellitus or complications associated with diabetes mellitus such as diabetic retinopathy, 
         [0013]    wherein in a further embodiment the pharmaceutically active compound comprises at least one human insulin or a human insulin analogue or derivative, glucagon-like peptide (GLP-1) or an analogue or derivative thereof, or exedin-3 or exedin-4 or an analogue or derivative of exedin-3 or exedin-4. 
         [0014]    Insulin analogues are for example Gly(A21), Arg(B31), Arg(B32) human insulin; Lys(B3), Glu(B29) human insulin; Lys(B28), Pro(B29) human insulin; Asp(B28) human insulin; human insulin, wherein proline in position B28 is replaced by Asp, Lys, Leu, Val or Ala and wherein in position B29 Lys may be replaced by Pro; Ala(B26) human insulin; Des(B28-B30) human insulin; Des(B27) human insulin and Des(B30) human insulin. 
         [0015]    Insulin derivates are for example B29-N-myristoyl-des(B30) human insulin; B29-N-palmitoyl-des(B30) human insulin; B29-N-myristoyl human insulin; B29-N-palmitoyl human insulin; B28-N-myristoyl LysB28ProB29 human insulin; B28-N-palmitoyl-LysB28ProB29 human insulin; B30-N-myristoyl-ThrB29LysB30 human insulin; B30-N-palmitoyl-ThrB29LysB30 human insulin; B29-N-(N-palmitoyl-Y-glutamyl)-des(B30) human insulin; B29-N-(N-lithocholyl-Y-glutamyl)-des(B30) human insulin; B29-N-(ω-carboxyheptadecanoyl)-des(B30) human insulin and B29-N-(ω-carboxyheptadecanoyl) human insulin. 
         [0016]    Exendin-4 for example means Exendin-4(1-39), a peptide of the sequence H-His-Gly-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Leu-Ser-Lys-Gln-Met-Glu-Glu-Glu-Ala-Val-Arg-Leu-Phe-Ile-Glu-Trp-Leu-Lys-Asn-Gly-Gly-Pro-Ser-Ser-Gly-Ala-Pro-Pro-Pro-Ser-NH2. 
         [0017]    Exendin-4 derivatives are for example selected from the following list of compounds: 
         [0000]                        H-(Lys)4-des Pro36, des Pro37 Exendin-4(1-39)-NH2,                   H-(Lys)5-des Pro36, des Pro37 Exendin-4(1-39)-NH2,               des Pro36 [Asp28] Exendin-4(1-39),               des Pro36 [IsoAsp28] Exendin-4(1-39),               des Pro36 [Met(O)14, Asp28] Exendin-4(1-39),               des Pro36 [Met(O)14, IsoAsp28] Exendin-4(1-39),               des Pro36 [Trp(O2)25, Asp28] Exendin-4(1-39),               des Pro36 [Trp(O2)25, IsoAsp28] Exendin-4(1-39),               des Pro36 [Met(O)14 Trp(O2)25, Asp28] Exendin-4(1-39),               des Pro36 [Met(O)14 Trp(O2)25, IsoAsp28] Exendin-4(1-39);       or               des Pro36 [Asp28] Exendin-4(1-39),               des Pro36 [IsoAsp28] Exendin-4(1-39),               des Pro36 [Met(O)14, Asp28] Exendin-4(1-39),               des Pro36 [Met(O)14, IsoAsp28] Exendin-4(1-39),               des Pro36 [Trp(O2)25, Asp28] Exendin-4(1-39),               des Pro36 [Trp(O2)25, IsoAsp28] Exendin-4(1-39),               des Pro36 [Met(O)14 Trp(O2)25, Asp28] Exendin-4(1-39),               des Pro36 [Met(O)14 Trp(O2)25, IsoAsp28] Exendin-4(1-39),            
wherein the group -Lys6-NH2 may be bound to the C-terminus of the Exendin-4 derivative;
 
or an Exendin-4 derivative of the sequence
 
         [0000]                        H-(Lys)6-des Pro36 [Asp28] Exendin-4(1-39)-Lys6-NH2,                   des Asp28 Pro36, Pro37, Pro38Exendin-4(1-39)-NH2,               H-(Lys)6-des Pro36, Pro38 [Asp28] Exendin-4(1-39)-NH2,               H-Asn-(Glu)5des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-NH2,               des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36 [Trp(O2)25, Asp28] Exendin-4(1-39)-Lys6-NH2,               H-des Asp28 Pro36, Pro37, Pro38 [Trp(O2)25] Exendin-4(1-39)-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Trp(O2)25, Asp28] Exendin-4(1-39)-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Trp(02)25, Asp28] Exendin-4(1-39)-NH2,               des Pro36, Pro37, Pro38 [Trp(O2)25, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Trp(O2)25, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Trp(O2)25, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36 [Met(O)14, Asp28] Exendin-4(1-39)-Lys6-NH2,               des Met(0)14 Asp28 Pro36, Pro37, Pro38 Exendin-4(1-39)-NH2,               H-(Lys)6-desPro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-NH2,               des Pro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-Asn-(Glu)5 des Pro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-Lys6-des Pro36 [Met(O)14, Trp(O2)25, Asp28] Exendin-4(1-39)-Lys6-NH2,               H-des Asp28 Pro36, Pro37, Pro38 [Met(O)14, Trp(02)25] Exendin-4(1-39)-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Met(O)14, Asp28] Exendin-4(1-39)-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28] Exendin-4(1-39)-NH2,               des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28] Exendin-4(1-39)-(Lys)6-NH2,               H-(Lys)6-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28] Exendin-4(S1-39)-(Lys)               6-NH2,               H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28] Exendin-4(1-39)-               (Lys)6-NH2;            
or a pharmaceutically acceptable salt or solvate of any one of the afore-mentioned Exedin-4 derivative.
 
         [0018]    Hormones are for example hypophysis hormones or hypothalamus hormones or regulatory active peptides and their antagonists as listed in Rote Liste, ed. 2008, Chapter 50, such as Gonadotropine (Follitropin, Lutropin, Choriongonadotropin, Menotropin), Somatropine (Somatropin), Desmopressin, Terlipressin, Gonadorelin, Triptorelin, Leuprorelin, Buserelin, Nafarelin, Goserelin. 
         [0019]    A polysaccharide is for example a glucosaminoglycane, a hyaluronic acid, a heparin, a low molecular weight heparin or an ultra low molecular weight heparin or a derivative thereof, or a sulphated, e.g. a poly-sulphated form of the above-mentioned polysaccharides, and/or a pharmaceutically acceptable salt thereof. An example of a pharmaceutically acceptable salt of a poly-sulphated low molecular weight heparin is enoxaparin sodium. 
         [0020]    Pharmaceutically acceptable salts are for example acid addition salts and basic salts. Acid addition salts are e.g. HCl or HBr salts. Basic salts are e.g. salts having a cation selected from alkali or alkaline, e.g. Na+, or K+, or Ca2+, or an ammonium ion N+(R1)(R2)(R3)(R4), wherein R1 to R4 independently of each other mean: hydrogen, an optionally substituted C1-C6-alkyl group, an optionally substituted C2-C6-alkenyl group, an optionally substituted C6-C10-aryl group, or an optionally substituted C6-C10-heteroaryl group. Further examples of pharmaceutically acceptable salts are described in “Remington&#39;s Pharmaceutical Sciences” 17. ed. Alfonso R. Gennaro (Ed.), Mark Publishing Company, Easton, Pa., U.S.A., 1985 and in Encyclopedia of Pharmaceutical Technology. 
         [0021]    Pharmaceutically acceptable solvates are for example hydrates. 
         [0022]    The term “drug delivery device” according to the invention shall mean a device designed to dispense a dose of a medicinal product, preferably multiple selected doses, e.g. of insulin, growth hormones, low molecular weight heparins, and their analogues and/or derivatives etc. Said device may be of any shape, e.g. compact or pen-type. Dose delivery may be provided through a mechanical, electrical or electro-mechanical dosing mechanism or by a stored energy dosing mechanism, such as a spring, etc. Furthermore, said device may comprise a needle or may be needle-free. Preferably, the term “drug delivery device” shall mean a reuseable multi-dose pen-type device having mechanical and manual dose selection and dose delivery mechanisms, which is designed for regular use by persons without formal medical training such as patients. Preferably, the drug delivery device is of the injector-type. Most preferably the drug delivery device is designed to deliver a fluid drug. 
         [0023]    The term “piston rod” according to the invention shall mean a device driving a piston, which is used to expel a substance or fluid from the receptacle. The dimensions of the piston rod may comprise a longitudinal major extension or axis, which is sufficiently large to enable an axial translational movement corresponding to the travel of the piston. 
         [0024]    The term “stop element” has been chosen to distinguish this element from further mechanical elements of the device and shall, in conjunction with the invention, generally mean a mechanical element or component that is designed to be able to inhibit an undesired movement of the piston rod during the reset operation as it is described below. The shape of the stop element can be adapted to the requirements of individual embodiments of the drug delivery device without departing from the scope of the invention. 
         [0025]    An embodiment of the drug delivery device comprises a body having a proximal and a distal end, a receptacle provided for a drug or pharmaceutical fluid near the distal end of the body, a piston rod which is movable in an axial direction from the proximal end towards the distal end for drug delivery, and a drive mechanism for the advancement of the piston rod. 
         [0026]    A reset mechanism is provided for resetting the piston rod to a start position at the proximal end when the receptacle is empty. 
         [0027]    The receptacle can be provided to be filled by inserting a cartridge containing the drug and a piston, which is to be moved by the piston rod. A receptacle provided for a cartridge is refilled by removing an emptied cartridge, resetting the piston rod, and inserting a new cartridge. Furthermore, if the receptacle is provided to be refilled by inserting a cartridge containing the drug and a piston, an undesired movement of the piston is avoided in the course of the reset operation by means of the stop element. 
         [0028]    In a further embodiment of the drug delivery device, the reset operation is merely generated by the weight of the piston rod. The reset operation is started by a release of the piston rod, so that the piston rod is free to move. If the device is then held with its proximal end pointing downwards, the gravitation makes the piston rod return to a start position at the proximal end. This movement may be accompanied with a rotation of the piston rod relatively to the body of the device and may especially be a helical movement generated by a screw thread of the piston rod. 
         [0029]    The materials of the piston rod and further elements of the mechanism on which the piston rod slides during the reset operation may be selected in view of making these components sufficiently smooth-running on one another, and the surfaces are given a finish that is most suitable to this purpose. If the device is held with its proximal end pointing down, the downward force exerted by the piston rod due to its weight may suffice to generate a sliding movement of the piston rod back into the drive mechanism. A dedicated reset element is therefore not necessary. 
         [0030]    In a further embodiment of the drug delivery device, the movement of the piston rod during drug delivery is guided relatively to the body by a guide element. To reset the piston rod, it is released from the guidance relatively to the body, so that the piston rod is free to move axially through the body, irrespective of a rotation. If the body is handled in a manner that the piston rod might be able to move from the start position, to which it is reset, back towards the distal end of the body, such a movement is blocked by means of the stop element. 
         [0031]    The stop element is preferably fixed with respect to the body during the drug delivery, and a coupling device is preferably provided between the guide element and the stop element. When the coupling device is engaged, the movement of the piston rod is guided relatively to the body. When the reset is started, the stop element is only rotationally fixed relatively to the body, the coupling device is disengaged, and the guide element can move freely relatively to the stop element as long as the piston rod is moving towards the proximal end to attain the start position. If the piston rod should move back towards the distal end, it shifts the guide element in the same direction, and the guide element is automatically engaged with the stop element by means of the coupling device and guides the motion of the piston rod again. The guide element may shift the stop element a small distance towards the distal end, until the axial movement of the stop element is stopped relatively to the body by a suitable device, which may be a protruding structure of the body, for example, and the guide element is simultaneously stopped. The movement of the piston rod is restricted by the guide element in such a manner that a movement of the piston rod towards the distal end is inhibited when the guide element is locked to the body by means of the stop element and the coupling device. Thus the stop element inhibits an undesired movement of the piston rod. The maximal distance which the stop element can axially move may be adapted by the placement of the means for stopping the stop element, the protruding structure of the body, for example, and may be chosen to be very small. Thus an undesired movement of the piston rod can be stopped almost immediately. 
         [0032]    In a further embodiment of the drug delivery device a mechanism is provided, by which the doses to be administered can be preset. A mechanism of this kind may be constructed according to the following description, for example. The piston rod comprises a screw thread at the distal end and a further screw thread at the proximal end, the screw threads having different pitches and opposite senses of rotation. The first screw thread is provided for the generation of a helical movement of the advancing piston rod, during the delivery of a preset dose, by means of a piston rod nut, which is kept fixed relatively to the body. The further screw thread is provided for a threaded engagement with a drive sleeve of the drive mechanism. During the setting of a dose the piston rod is prevented from rotation and axial translation relatively to the body by the combined effect of the thread of the fixed piston rod nut and the thread of the drive sleeve, which moves helically around the piston rod without changing the position of the shape of the surface that faces the further screw thread of the piston rod. The thread of the drive sleeve therefore appears during the set operation as if it were fastened to the body, apart from the sliding movement on the surface of the piston rod. 
         [0033]    When a reset operation is started, the piston rod nut is released from the body. This may be effected by removing the empty cartridge, for example. The piston rod is now able to rotate relatively to the body like a screw into the drive sleeve, until a start position is attained. Furthermore, the piston rod nut is also allowed to rotate freely in order to compensate for a discrepancy between the actual helical movement of the piston rod and a helical movement that would be generated by the thread of the piston rod nut if it were still fixed relatively to the body. After the release of the piston rod nut a translational movement of the piston rod in the axial direction towards the proximal end is therefore possible independently of the pitch of the thread of the piston rod nut. 
         [0034]    In an embodiment of the drug delivery device, the piston rod nut is the guide element and is fixed relatively to the body by a lock nut, which is the stop element. The lock nut is engaged with the piston rod nut by means of a coupling device during drug delivery. The coupling device can be realized, for instance, by a gear formed by surface structures of the nuts. A spring is arranged between the lock nut and a part of the body in such a manner that the spring tends to disengage the lock nut and the piston rod nut. When a reset operation is started. the lock nut is shifted towards the distal end by the force of the spring and disengaged from the piston rod nut. The piston rod nut is free to rotate, and the piston rod is able to move axially without further guidance by the piston rod nut. The movement of the lock nut towards the distal end is inhibited by a rim or similar structure within the body. If the piston rod should move back towards the distal end, the piston rod nut is shifted towards the lock nut, and the nuts are again engaged by the coupling device. The guidance of the motion of the piston rod is thus reestablished, and the further movement towards the distal end blocked by the arrested lock nut. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0035]    Further aspects and examples of the invention are described in conjunction with the appended figures. 
           [0036]      FIG. 1  shows a cross-section of an injection pen having a mechanism comprising a reset operation. 
           [0037]      FIG. 2  shows an enlarged cross-section of a part of the mechanism that is involved in the reset operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  shows a cross-section of an injection device  1  in the shape of a pen having a proximal end  2 , a distal end  3  and a housing or body  4  carrying a needle  5  at the distal end  3 . A receptacle  6  serving as a reservoir for a drug that is to be injected through the needle  5  is provided at the distal end  3  and can be refilled. The delivery of the drug is effected by means of a piston  7 , which is moved by a piston rod  8  in the axial direction, along the longitudinal extension of the device, thus reducing the volume of the reservoir according to the doses to be administered. The receptacle  6  can be provided for the insertion of a cartridge  16  containing the drug. In this case, the piston  7  is moved in the cartridge  16  and the piston rod  8  moves through a hole in the bottom of the cartridge  16 . At the distal end  3  the body  4  comprises a distal part  20 , which can be taken off from a proximal part  21  of the body  4 , so that the cartridge  16  can be removed and substituted with another one. 
         [0039]    The piston rod  8  carries a screw thread  9  at its distal end and passes through a central hole of a piston rod nut  14 , which has a thread of the same pitch on the inner wall of its central hole. The piston rod  8  and the piston rod nut  14  are interlocked by the screw thread  9  and can be rotated relatively to one another. Simultaneously with the rotation, the screw thread  9  generates an axial relative movement resulting in an overall helical relative movement. The piston rod  8  and the piston rod nut  14  thus form a pair of sliding elements. The friction between these elements is reduced if they are formed from low-friction synthetic materials. 
         [0040]    A lock nut  13  is engaged with the piston rod nut  14  by means of a coupling device. A spring  15  is arranged between the lock nut  13  and a barrier rim  27  or transverse inner wall of the body  4 . The coupling device can be realized, for instance, by a gear formed by surface structures of the nuts  13 ,  14 . When the injection pen  1  is ready for use, the spring  15  is compressed by the cartridge  16  inserted in the receptacle  6 , and the coupling device engages the nuts  13 ,  14 . The compressed spring tends to shift the lock nut  13  towards the distal end  3  and thereby disengage the lock nut  13  and the piston rod nut  14 . 
         [0041]      FIG. 2  shows an enlarged view of the arrangement of the lock nut  13  and the piston rod nut  14 . In the center of  FIG. 2 , a section of the piston rod  8  is shown. An imaginary central axis  18  is indicated by the broken line of alternating dots and dashes. The piston rod  8  goes through a hole  23  of the lock nut  13 , a hole  24  of the piston rod nut  14  and a hole  26  in the bottom of the cartridge  16 , which is inserted in the receptacle  6 . The spring  15 , which may be a helical spring, surrounds the piston rod  8  and is arranged between the lock nut  13  and the barrier rim  27 , which can be an integral part of the body  4 . The outermost surfaces of the spring  15  are in contact with the surfaces of the lock nut  13  and the barrier rim  27  facing the spring  15 . 
         [0042]    The coupling device  17  engaging the lock nut  13  and the piston rod nut  14  can be formed by a sequence of interlocking teeth or some other kind of gear, for example. This is indicated in  FIG. 2  by the lock nut  13  partially intruding the outer margin of the piston rod nut  14  in the area of the coupling device  17 . The lock nut  13  is engaged with the body  4 , by means of protruding parts or recesses, for example, so that the lock nut  13  cannot rotate relatively to the body  4  around the axis  18  of the piston rod  8 , but can axially move at least a small distance towards the distal end  3 . In the embodiment shown in  FIG. 2  this is effected by grooves  25  forming internal splines in the inner sidewall of the body  4 , which guide external splines on the circumference of the lock nut  13 . The grooves  25  preferably extend only a short way towards the distal end  3 , forming stops  22  at their end. An axial movement of the lock nut  13  towards the distal end  3  is inhibited by the presence of the cartridge  16  and is blocked by the stops  22  else. An axial movement of the nuts  13 ,  14  in the direction towards the proximal end  2  can be inhibited by barrier rims  27 , spikes or a partial transverse wall of the body  4 , for example, fixed at the inner surface of the body  4  or forming an integral part of the body  4  on the side of the nuts  13 ,  14  facing away from the piston  7 . 
         [0043]    The hole  24  of the piston rod nut  14  is supplied with a thread  19  having the same pitch as the screw thread  9  of the piston rod  8 . Thus, the thread  19  of the piston rod nut  14  is the female thread counterpart of the male screw thread of the piston rod  8 . If the nuts are engaged by the coupling device  17  as shown in  FIG. 2 , a relative rotation of the lock nut  13  and the piston rod nut  14  is inhibited, and the piston rod nut  14  cannot rotate relatively to the body  4  around the piston rod  8  because the lock nut  13  is rotationally fixed by the protruding parts, recesses or grooves  25 . 
         [0044]    If the cartridge  16  is removed and the spring  15  is released, the coupling device  17  is no longer interlocked, and the lock nut  13  and the piston rod nut  14  are disengaged. This means that the piston rod nut  14  can freely rotate around the piston rod  8 , and the piston rod  8  is able to perform a translational movement along its axis  18  irrespective of a rotation around its axis  18 . This is, because the piston rod nut  14  will freely rotate and compensate for a discrepancy between the actual translational movement of the piston rod  8  and a translational component of a helical movement of the piston rod  8  that would be generated by the threads  9 ,  19  when the piston rod nut  14  is rotationally fixed. 
         [0045]    The operations of setting and delivering a dose will now be described in conjunction with  FIG. 1 , which shows, by way of example, an embodiment that comprises means for setting a dose, including the piston rod  8  and further components arranged in the proximal part  21  of the body  4 . The piston rod  8  is provided with a further screw thread  10  at its proximal end. A drive sleeve  11  having an inner thread  12  of the same pitch is interlocked with the further screw thread  10  of the piston rod  8 . A dial sleeve  28  having a helical thread engaged with a helical thread  29  of the body  4  and having the same pitch as the further screw thread  10  of the piston rod  8  is arranged around the drive sleeve  11  and can be rotated by the user by means of a dial grip  36 , which surpasses the proximal end  2  of the body  4 , for example. A clutch  30  of essentially cylindrical shape is arranged between the drive sleeve  11  and the dial sleeve  28 . 
         [0046]    The drive sleeve  11  is provided with a kind of rim or collar  31  at its distal end. A spring  32  is arranged between the collar  31  and the clutch  30  and tends to press the clutch  30  against the inner surface of a proximal endface of the dial sleeve  28 , where a gear  37  or similar surface structure is provided to inhibit, when engaged, a relative rotation of the clutch  30  with respect to the dial sleeve  28 . A translational movement of the drive sleeve  11  relatively to the clutch  30  can preferably be limited by hooks  35  of the drive sleeve  11 , which are stopped by the proximal end of the clutch  30 . A relative rotation of the clutch around the drive sleeve  11  is permanently inhibited, for instance, by clutch leads  33  engaging a gear  34  at the proximal end of the drive sleeve  11  in such a manner that the clutch  30  can be shifted a short distance relatively to the drive sleeve  11  towards the distal end of the drive sleeve  11  against the force of the clutch spring  32 . This shift can be effected by means of an operation button  39  arranged at the proximal end  2  of the body  4  in contact with an end face  38  of the clutch. If the operation button  39  is pressed, the clutch  30  is shifted towards the distal end  3  and disengaged from the gear  37  of the dial sleeve  28 , so that a relative rotation between the clutch  30  and the dial sleeve  28  is made possible. 
         [0047]    During the set operation to select a dose of the drug to be delivered, the operation button  39  is not pressed and the dial sleeve  28  and the clutch  30  are rotationally coupled by the gear  37 . By rotating the dial grip  36 , the dial sleeve  28  is moved out of the body  4  in a helical movement generated by the thread  29 . Because of the rotational coupling, the dial sleeve  28 , the clutch  30  and the drive sleeve  11 , forming a dial assembly, always rotate by the same angle. Because the threads of the drive sleeve  11  and the dial sleeve  28  have the same pitch, the position of the shape of the surface that faces the further screw thread  10  of the piston rod  8  does not change during the helical movement of the dial assembly. The piston rod  8  thus always sees the same spatial position of the inner thread of the drive sleeve  11 , as if this thread were an integral part of the body  4 . The piston rod nut  14  is fixed during the set operation, so that its thread  19  also appears as if it were an integral part of the body  4 . Since the two threads  9 ,  10  of the piston rod  8  have different pitches, and also different senses of rotation, the different helical movements corresponding to the different pitches cannot be performed simultaneously by the stiff piston rod  8 , which is therefore fixed relative to the body  4  and neither moves in axial direction nor rotates. The rotation of the dial assembly takes therefore place without changing the relative axial position of the dial sleeve  28  and the drive sleeve  11 , so that the clutch  30  remains engaged with the dial sleeve  28  by means of the gear  37 . 
         [0048]    After the desired dose has been set, it may be delivered by pressing the operation button  39  and moving the piston rod  8  towards the distal end  3 . The operation button  39  extends through the dial grip  36  and is in contact with a proximal end face  38  or other extreme end part of the clutch  30 . When the operation button  39  is pressed, clutch  30  is axially shifted towards the distal end  3  of the device with respect to the dial sleeve  28 , thereby decoupling the clutch  30  from the dial sleeve  28 . However, the clutch  30  remains keyed in rotation to the drive sleeve  11 . Therefore the decoupling of the clutch  30  results in a decoupling of the dial sleeve  28  and the drive sleeve  11 . The dial sleeve  28  is free to rotate in a helical movement according to the thread  29 , thus moving back into the body  4  to its original position. The clutch  30  is being kept from rotation either by the operation button  39  directly, for instance, or by some other means not shown in  FIG. 1 , which may be arranged between an outer structure of the clutch  30  and axial grooves in the body  4 , for instance, in order to guide the movement of the clutch, allowing an axial translation but inhibiting a rotation. The drive sleeve  11 , which cannot be rotated relatively to the clutch  30 , is therefore not wound around the piston rod  8  as during the set operation, but shifts the piston rod  8  towards the distal end  3 . The piston rod  8  performs a helical movement with respect to the body  4  through the piston rod nut  14 . Because of the different senses of rotation of the screw threads  9 ,  10  of the piston rod  8 , the drive sleeve  11  is moved towards the piston rod nut  14 , and the axial displacement of the drive sleeve  11  exceeds the displacement of the piston  7 , which is defined by the axial translation of the piston rod  8  relatively to the body  4 . The different pitches of the screw threads  9 ,  10  of the piston rod  8  define the relation between the distance that the dial assembly is axially moved per unit dose during the set operation and the distance that the piston  7  is moved during the delivery of one unit dose, and the pitches can be adjusted to provide a desired transmission for easy use. The pitch of the thread of the drive sleeve  11 , corresponding to the further screw thread  10  of the piston rod  8 , is preferably larger than the pitch of the thread of the piston rod nut  14 . 
         [0049]    When the final dose has been dispensed, the cartridge  16  is empty and may be removed. To this end, the distal part  20  of the body is taken off together with the cartridge  16 , and the lock nut  13  is released. The spring  15  moves the lock nut  13  away from the piston rod nut  14 , so that the nuts  13 ,  14  are no longer engaged by the coupling device  17 . Then the piston rod nut  14  can rotate relatively to the body  4  and is ready for the reset operation. 
         [0050]    During reset, the piston rod  8  is screwed into the drive sleeve  11  towards the proximal end  2 . To this purpose it may suffice to hold the injection device with its proximal end  2  downwards and have the piston rod  8  rotate into the drive sleeve  11 , following the gravitation of its own weight. In order to achieve this, the materials of the sliding elements are selected to be sufficiently smooth-running. 
         [0051]    The friction between rough planar surfaces of two bodies that are in contact and move relatively to one another, so that the surfaces slide on one another, generates a force F R  of a retarding effect directed within the plane of the surfaces, thus decreasing the velocity of the relative movement. At a certain specified relative speed of the bodies, the absolute value of the frictional force F R  can generally be regarded as being proportional to the absolute value of a force F N  perpendicular to the plane of the surfaces, by which the bodies are pressed on one another. The quotient of the absolute value of the frictional force F R  and the absolute value of the perpendicular force F N  is called coefficient μ of sliding friction, so that the equality F R =μ(v r )×F N  is supposed for any specified relative velocity v r  of the bodies. 
         [0052]    If the mechanical elements are made of synthetic or plastic material, a value of the coefficient μ of sliding friction of typically 0.25 or less at a relative velocity of the sliding surfaces of 2 mm per second can be obtained. The piston rod  8  may be a liquid crystalline polymer and the piston rod nut  14  and the drive sleeve  11  polyoxymethylene, for example. A comparable value of μ can be obtained for the sliding movement of the piston rod nut  14  on a surface or barrier rim  27  of a body  4  made of synthetic or plastic material. 
         [0053]    The properties of the materials can also be specified by the roughness of their surfaces. The roughness can be measured by the deviation of the actual rough surface from a smooth reference surface. A characteristic parameter of the roughness is the average of the absolute value of the distance between a point of the actual surface and the corresponding point of the reference surface, called centre line average. The materials of the piston rod  8 , the piston rod nut  14  and the drive sleeve  11  are preferably selected to have a centre line average of less than 1.6 μm. 
         [0054]    A further parameter that is relevant with respect to the ease of the sliding motion of the piston rod  8  through the threads is the pitch of the threads. A pitch of a screw thread can be specified, depending on the diameter of the screw, by the angle between the axis of the screw and the tangents of the helix formed by the thread. The reset operation is promoted if the angle between the axis  18  of the piston rod  8  and the tangents of the helices of the threads  9 ,  10  is as large as feasible. This angle can be made at least 25° for the thread of the piston rod nut  14  and at least 40° for the thread of the drive sleeve  11 . Larger values may be favourable to the reset operation, but upper limits of the angles are imposed by the shape and dimensions of the device. 
         [0055]    Although it is preferred to have smooth-running surfaces to facilitate the reset operation, the low friction may cause a reversal of the movement of the piston rod when the device is turned. This may occur during the process of refilling the receptacle  6  or inserting a new cartridge  16 . When the reset is started by removing the cartridge  16 , the spring  15  moves the lock nut  13  away from the piston rod nut  14 , and the coupling device  17  is disengaged. The piston rod nut  14  is then free to rotate around the piston rod, the guidance of the motion of the piston rod  8  by the piston rod nut  14  is removed, and the piston rod  8  can perform a suitable helical movement to the start position. If the device is turned with the distal end  3  pointing downwards, the gravitation acting on the weight of the piston rod  8  will make the piston rod  8  move back out of the drive sleeve  11  towards the distal end. The piston rod nut  14  moves together with the piston rod  8 , because it is engaged by the interlocking screw thread  9 . The movement of the piston rod nut  14  is stopped by the lock nut  13 , which is stopped at the end of the groove  25  and then blocks the way of the piston rod nut  14 . When the piston rod nut  14  touches the lock nut  13 , the coupling device  17  engages the nuts  13 ,  14  again. The piston rod nut  14  is therefore prevented from a further rotation, and the guidance of the motion of the piston rod  8  relatively to the body  4  is reestablished. Since the helical movement of the piston rod  8  out of the drive sleeve is not in accordance with the screw thread  9  the helical movement of the piston rod  8  is inhibited, so that the piston rod  8  essentially stays at the start position. 
         [0056]    The stop element has been described in detail in conjunction with embodiments in order to make the function of the stop element completely clear. Nevertheless, the stop element and the appertaining mechanism can be realized in different ways without departing from the spirit and scope of the invention.