Abstract:
A pen injection device designed for use with dual or multi-chamber cartridges  14 , is provided with an extended length plunger  24  having a non-drive portion  24 ′ on its forward end and the usual screw-threaded drive portion  24 ″ on its rearward end. This arrangement allows the plunger to be pushed forwardly while the dosing drive mechanism is disengaged to effect a reconstitution movement of the rearward bung  18 ′ in the cartridge. Thereafter the threaded portion on the plunger  24 ′ engages the drive mechanism to allow metered dosing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 12/300,174 filed on Dec. 9, 2008 and issued as U.S. Pat. No. 8,167,834 on May 1, 2012; which is the 35 U.S.C. 371 national stage of International application PCT/GB07/01721 filed on May 10, 2007; which claims priority to British application 0609311.6 filed on May 11, 2006. The entire contents of each of the above-identified applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to an injection device and in particular, but not exclusively, to an injection device for use with dual or multi-chamber cartridges. 
     DESCRIPTION OF THE RELATED ART 
     In typical dual chamber cartridges two spaced bungs are disposed within the cartridge to define two separate chambers, for example a forward chamber containing lyophilised powder and a rearward chamber containing diluent liquid. A bypass channel in the cartridge wall provides a bypass path between the two chambers, but is initially closed by the presence of the forward bung. In use, as pressure is applied to the rearward bung to move it, the liquid in the diluent chamber and the forward bung move together forwardly and in doing so uncover the bypass channel so that the fluid from the diluent chamber flows into the forward chamber to mix with the lyophilised powder. The forward bung and the rearward bung may be of similar form or the forward item may be in the form of a moveable wall. 
     In such cartridges, it will be appreciated that two phases of movement of the rearward bung are required, namely a first, reconstitution phase during which the rearward bung moves to move the liquid in the rearward chamber forward to mix with the material in the forward chamber, and a second, dosing phase during which metered movement of the rearward bung is required to expel a preset dose. 
     In a typical pen injector such as our Autopen® device, the pen injector comprises a main body housing the pen injector mechanism which screws into a cartridge holder which holds the cartridge of therapeutic substance. In use the user dials in a required number of units dosage on a dose selector on the body and then presses the release button which causes an internal plunger in the body in contact with the bung to advance it by the required amount to expel a corresponding dose from the cartridge in the cartridge holder. 
     A problem arises in relation to such devices in relation to use with dual chamber cartridges because the amount of movement required during the reconstitution phase 15-20 mm or more is much greater than that required for the doses (typically 0.1 to 0.2 mm per unit dose). If the pen injector mechanism were to be used to try and achieve the length of stroke required for the reconstitution phase this would be extremely tedious requiring many cycles of repetitive dialing in and firing. A previous attempt to overcome this problem provided an pen injector in which the body and the cartridge holder had extended threaded mating sections so that the holder could be engaged with the body and screwed continuously until the plunger had driven the rearward bung by an amount sufficient for the reconstitution phase. Thereafter the pen injector mechanism could be operated as required to effect a series of doses. This extended screw thread arrangement is also extremely tedious and time consuming. 
     There is therefore a need for an injection device for use with a multi-chamber cartridge, to allow convenient reconstitution followed by the usual dose metering in successive injections. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, this invention provides an injection device for dispensing in use a substance from a dual or multi-chamber cartridge, said cartridge having a bung moveable in a first, reconstitution, extent of movement and a second, dosing, extent of movement (which may be variable), said device comprising: 
     a body portion; 
     a plunger mounted in said body portion for longitudinal movement with respect thereto in a forward dispensing direction to co-operate in use with said bung; 
     a plunger drive element disposed adjacent said plunger for driving said plunger; 
     said plunger having a drive portion adapted to mate with a complementary drive portion on said plunger drive element and adapted to impart dosing movement to said plunger; 
     said drive portion extending only partly along said plunger to leave a forward non-drive portion; 
     whereby, during a reconstitution phase, said plunger may be moved longitudinally forwards from a position with the non-drive portion adjacent said plunger drive element, to bring its drive portion into driving engagement with the drive portion on said plunger drive element. 
     In this manner, the plunger may be moved to effect reconstitution of the therapeutic mixture in the cartridge before the drive portions are engaged. This allows a separate reconstitution movement followed by a metered dosing movement of the plunger, without compromising the requirements of either movement. 
     Preferably, said drive portions on said drive element and said plunger are complementary threaded drive portions. Thus the plunger may carry a male thread and the drive element may include a bore having a female thread therein although the reverse is not excluded. In this instance, the non-drive portion of said plunger may comprise a non-threaded portion sized to be moveable freely through the drive element until the drive portions engage. 
     The initial reconstitution movement of the plunger may be achieved in various ways. In one arrangement a mover element is slideably moveable in said body portion and has an externally accessible push region so that the element may be manually pushed to engage said plunger and to move it longitudinally until said complementary drive portions are in driving engagement. The device preferably includes means for releasably preventing longitudinal movement of said mover element. This may typically comprise a bayonet type lock arrangement between the mover element and an element fixed with respect to the body portion. In addition, or alternatively, it may comprise a releasable spacer element. 
     Conveniently, the mover element includes a cap element defining the push region, said cap element being adapted to engage in snap engagement in an end region of said injection device. 
     Preferably, the device includes bias means for urging the plunger element towards complementary driving engagement with said drive element. Said bias means and said plunger are preferably arranged such that said plunger is isolated from the influence of the bias means a preset distance after initial driving engagement between said complementary drive portions. 
     The drive to the drive element may be achieved in various ways and in one embodiment, it is in the form of a drive mechanism which applies a pre-selectable amount of angular movement to said drive element on activation thereof. Thus the drive mechanism may comprise a user settable dose setting member rotatably mounted on said body portion to set a dose, with a spring acting between the dose setting member and the body portion, said spring being strained to a variable extent depending on movement of said dose setting member. A ratchet arrangement may be disposed between the dose setting member and said drive element, allowing one way relative movement therebetween as the dose setting member is rotated to set a dose. A trigger member may be provided to prevent rotary movement of the drive element during the dose setting routine but to be releasable thereafter to allow said spring to rotate said dose setting member and said drive element in unison by an amount corresponding to the set dose. In this arrangement, the dose setting element preferably includes a generally cylindrical portion surrounding at least part of the plunger. 
     The dose setting element may conveniently comprise a rotatable collar or the like on the rearward end of the injection device and, where provided, the mover element may be arranged to snap fit into said collar when the mover element is moved fully forward. 
     Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description. 
    
    
     
       BRIEF DESCRIPTION OF VIEWS OF THE DRAWINGS 
       The invention may be formed in various ways, and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings in which: 
         FIG. 1   a  is a schematic view of a pen injection device in accordance with this invention for use with a multi-chamber cartridge, before reconstitution; 
         FIG. 1   b  is an expanded view of the ratchet mechanism; 
         FIG. 2  is a schematic view similar to  FIG. 1  but during the latter stages of reconstitution and before a dose has been set, and 
         FIG. 3  is a schematic side view after a number of doses have been expelled. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings, a pen injection device is made up of a cartridge holder  10  screw-threaded onto a main body portion  12 . Within the cartridge holder  10  is provided a dual chamber cartridge  14  having two chambers  16 ′ and  16 ″ defined by rearward and forward bungs  18 ′,  18 ″. The rearward chamber  16 ′ normally contains a diluent liquid and the forward chamber  16 ″ normally contains a lyophilised powder. Part way along the syringe, and initially closed off by the forward bung  18 ″, is a bypass  20  channel disposed in the wall of the cartridge and interconnecting the forward and rearward chambers  16 ′,  16 ″. Screwed onto the end of the cartridge holder  10  is a double ended needle  22  which pierces a rubber membrane (not shown) in the front of the cartridge. 
     The rearward bung  18 ′ in use is acted upon and driven by a plunger  24 . The plunger  24  can move linearly under certain conditions but is prevented from rotation by means of two diametrically opposed channels (not shown) which cooperate with a correspondingly shaped washer  26  itself held against rotation on the forward end of the main body portion  12 . Unlike previous such devices, in which the plunger shaft has been threaded along its operational length, in this embodiment of the invention the forward two-thirds or so  24 ′ of the plunger  24  is plain, but to the rear of this is a threaded section  24 ″ which terminates in a flange  27  and a plain end  28 . Rotatably fitted in the rearward end of the body portion  12  is a dose setting collar  30  which is fixedly connected to a setting cylinder  32  the forward end of which is provided with an externally directed ratchet surface  33   a  which cooperates with an internally directed ratchet surface  33   b  on the drive gear  34 , as shown in  FIG. 1   b . The ratchet mechanism allows relative ratcheting movement of the dose setting collar  30  and the setting cylinder  32  relative to the drive gear  34  in a dose setting direction but not in the reverse, dose expelling direction. As drawn, the dose setting direction is clockwise when viewed on the rearward end of the device. A torsion spring  35  is secured at its rearward end to the dose setting collar and at its forward end is anchored to be rotationally fixed with respect to the main body  12 . A trigger  36  is slidably mounted in said main body portion  12  and moveable against a spring bias from a forward position in which it locks the drive gear  34  against rotation and a rearward position in which the drive gear  34  is free to rotate. 
     In use of the metered dose setting drive mechanism, the required dose is dialed in by rotating the dose setting collar  30  by the required number of clicks provided by the ratchet between the setting cylinder  32  and the drive gear  34 . During this setting routine, the drive sleeve  34  does not rotate, being kept still by the trigger  36  with the ratchet between the sleeve  32  and the drive gear  34  allowing relative movement between the two. Once the required number of clicks has been dialed in, the user then inserts the needle into the injection site and pushes the trigger  36  rearwardly to release the drive gear  34 . The drive gear  34 , now being free to rotate with the setting cylinder  32  and the setting collar  30 , is driven by the torsion spring anticlockwise by an amount equivalent to the angular amount dialed into the dose setting collar  30 . This rotary movement causes the plunger  24  to advance by virtue of its threaded engagement with the inside of the drive collar  34 . Further details of suitable arrangements are described in our U.S. Pat. No. 5,104,380. 
     As noted, in the embodiment of this invention, the plunger is threaded along only part of its length, the remainder being plain. The construction and operation of this device will now be described. In  FIG. 1   a , the plunger  24  is shown in its retracted position prior to driving the rearward bung  18 ′ forwards. Slideably mounted in the end of the setting cylinder  32  is a pushrod  38  with a cap  40 . The pushrod  38  is designed to be a sliding fit within the setting cylinder  32 . Releasing the pushrod  38  and pushing it causes it to engage the plain end  28  of the plunger  24  so that the plunger can be moved longitudinally in a reconstitution phase from the position shown in  FIG. 1   a  to that in  FIG. 2 . Once fully pressed in, the cap  40  on the pushrod  38  may snap-fit into the interior of the dose setting collar  30 . As seen in  FIG. 2 , the forward movement is sufficient to move the rearward bung  18 ′ towards the forward bung  18 ″ to allow mixing of the therapeutic mixture. Thus initially the rearward bung  18 ′ and the forward bung  18 ″, with the diluent liquid between them, move forward together a sufficient distance for the forward bung  18 ″ to uncover the bypass  20 , whereupon the forward bung remains stationary, as the diluent liquid enters the forward chamber, until the rearward bung abuts the forward bung. 
     A spring  42  acting between the rearward end of the setting cylinder  32  and the flange  27  on the plunger  24  lightly biases the plunger to urge the threaded portion  24 ″ on the plunger  24  into engagement with the corresponding female thread in the drive sleeve  34 . The pushrod  38  may interface with the setting sleeve in a bayonet lock type arrangement (not shown) requiring the cap  40  to be rotated before it can be pushed into the shell  32 , and also isolating the spring  42  and preventing the spring pushing the plunger onto the bung prior to use. 
     Following reconstitution, the device is made ready for injection by dialing in preset dose to prime the cartridge and to ensure that threads on the plunger and the drive collar are properly engaged. Thereafter the device may be used as a conventional pen injection device. 
     The relaxed length of the spring  42  is selected so that it ceases to apply pressure to the plunger  24  shortly after the threads on the plunger  24  and the drive sleeve  34  have engaged. If required a removable safety collar (not shown) may initially be releasably clipped around the exposed part of the pushrod  38  between the cap  40  and the dose setting collar  30 , to prevent premature mixing of the substances in the chambers of the cartridge.